Veterinary Diagnostic Services

The clinician must be aware of inherent limitations of laboratory evaluation in certain clinical settings. In general, veterinary diagnostic laboratories are preferred to general medical laboratories, because human medical laboratories may be less familiar with animal diseases and the responses of animals to disease. There may also be differences in test methodology and interpretation. These species differences can cause confusion when results are evaluated on the basis of human criteria that may not apply to animals.

Various desktop or portable hand-held point-of-care devices are available to veterinarians for determination of serum chemistry, electrolytes, and acid-base balance. Many of these devices use self-contained strips, cartridges, or rotors and thus reduce errors associated with the maintaining, measuring, and mixing of reagents. In addition, some of these devices can use whole blood rather than serum or plasma. Some devices require refrigerated storage of reagent cartridges, which must then be warmed to room temperature for use. There are relatively few independently published data comparing the results obtained with these point-of-care devices and those obtained with standard laboratory procedures. A widely used hand-held device (iSTAT) has been shown to yield comparable results for blood electrolyte concentrations and acid-base balance in dogs and horses.³ However, it was noted that the correlation’ between results from this device and from standard laboratory techniques was poor for sodium in the dog and for hematocrit in the horse. Portable point-of-care devices can provide rapid, accurate, and relatively inexpensive results. As technology continues to improve, it can be anticipated that there will be wider and more general application of these devices in many large animal practice settings. Important requirements are the establishment of normal values with these devices for our large animal species of important age or production groupings as well as clear definition of the limitations and possible idiosyncratic reactions in certain species or clinical settings.

Selection of Procedures

Selection of specific laboratory tests fosters logically integrated thinking and concentrates on evaluation of the primary medical problems. However, the sophisticated autoanalyzers used by large commercial laboratories can perform a wide battery of tests quickly and efficiently with little additional cost. These panels may be broadly defined (e.g., a general large animal health panel) or may offer a more focused evaluation of a specific organ system (e.g., liver, kidney, or muscle). The clinician must ensure that the panel selected contains all the appropriate tests for a thorough evaluation of the individual patient’s medical problems.

The following recommendations for diagnostic panels are intended to provide a clear indication of organ damage and/or dysfunction. The most directly applicable diagnostic procedures are listed under “Recommended,” and additional procedures that may be of benefit in certain circumstances are listed under “Optional.”

Metabolic Profiling

The health status and productivity of dairy cattle, swine, and other food animals maintained in large confined groups involve a fragile balance between metabolic events, nutrition, agents of disease, management, and environmental factors. In these production units the health status of the herd as a whole is of paramount importance. Subclinical disease or nutritional imbalance may contribute to suboptimal productivity. Most productivity problems in these settings are multifactorial. Defining and finding solutions to these problems can be a difficult and complicated task. Sequential assessment of weight gains, body conditions scores, milk quality, and milk production are useful measures of the presence of subclinical production disorders but do not identify the cause. Metabolic profiling is a tool that has been employed in some situations. Blood samples are drawn from a number of individuals as representative of the group as a whole. Some have recommended the submission of pooled serum samples from representative individuals, much like the use of bulk tank tests as a reflection of the general level of mastitis in a herd. Sampling may be done routinely and sequentially. In dairy cattle this may be done during gestation or lactation but frequently focuses on the periparturient period when a combination of nutritional and metabolic events often contributes to costly production disorders.

Metabolic profiles might include most of the parameters listed under the recommended general panel (p. 376), with the addition of magnesium, total cholesterol, nonesterified fatty acids (NEFA) and β-hydroxybutyrate (BOHB). BOHB, NEFA, and cholesterol may provide an indication of energy balance, whereas BUN, creatinine, total protein, albumen, and CK may be helpful in assessing protein status. In certain settings, trace minerals or fat-soluble vitamins may be important indicators of underlying nutritional problems. Metabolic profiling is not a substitute for careful clinical examination, analysis of husbandry practices, and ration analysis, but it may play a useful role in some modern large- scale operations in which a variety of subclinical problems can quickly translate into financial disaster.

Laboratory

One of the most commonly overlooked sources of variation in clinicopathologic data is the difference in results obtained by different laboratories. This can result in fivefold to tenfold differences in the normal range of certain enzyme activities between laboratories using similar but not identical methodologies. In the past there was marked variation in the units of measure used to express the activities of different serum enzymes. The standard method of representing serum enzyme activity is in international units per liter (IU/L), which is used in this text. Correction factors for converting the commonly used but older units of measure to international units are given in Table 22-1. The normal values given in Tables 22-2 and 22-3 are those used at the University of California Veterinary Medical Teaching Hospital or are from the literature.^2,3 It is always best to use normal values and reference intervals established for the species, age, and production type by the diagnostic laboratory to which samples are submitted.

Table 22-1 Conversion of Conventional Units to International Units

Table 22-2 Clinical Chemistry: Normal Range for Large Animals

Table 22-3 Serum Protein Electrophoresis: Normal Range for Large Animals

Species

There is relatively modest variation among species for most clinicopathologic parameters. Notable exceptions are plasma electrolyte concentration, erythrocyte potassium concentration in some breeds of sheep, and serum bilirubin concentration, which is higher in horses than in other species. BUN is a less reliable indicator of renal function in ruminants and horses than creatinine because urea nitrogen can be metabolized by the intestinal microflora. Donkeys and burros have a much higher GGT level than horses and cattle.

Breed

Significant differences in hematologic parameters exist between hot-blooded and cold-blooded horses. Hot-blooded horses include most of the athletic breeds of horses (the thoroughbred, quarter horse, standardbred, and Arabian breeds). Cold-blooded horses include the pony and draft breeds. Cold-blooded horses have lower red cell values both at rest and after exercise and maintain a slightly lower leukocyte count; they also have lower resting and fasting indirect bilirubin concentrations.

Age

There are several important differences in hematologic and clinical chemistry between neonatal and adult animals. The effects of age have been studied most carefully in horses and cattle. Suckling neonatal animals tend to have lower BUN, slightly lower total protein and globulin, moderately higher GGT and phosphate, and markedly greater alkaline phosphatase than do adult animals.

Sex

With the obvious exception of sex hormone concentrations, there are few recognized differences in clinical chemistry values between sexes. In most domestic animals the intact male tends to have a slightly higher erythrocyte count, hemoglobin concentration, and PCV than the female or neutered male. This sex-related difference has been demonstrated most clearly in the horse.

Factors Influencing Results or Their Interpretation

Many factors influence the reliability and interpretation of results obtained by laboratory analysis. Sample collection and handling are very important factors. The sample collection site (e.g., jugular vein, mammary vein, tail vein, or carotid artery) can have an important effect on the results of tests such as blood gas evaluation, glucose, or ketones. The choice of anticoagulants depends on whether the samples are to be submitted for serum, plasma, or whole blood determinations. The specific sample requirements for the most commonly ordered clinical chemistry determinations are listed in Table 22-4. Serum is required for most chemistry determinations, and serum separator tubes work very well in most settings. There have been some indications that results of some serum hormone assays may be influenced by collection of blood in serum separator tubes. Heparin is the anticoagulant of choice for most chemical determinations requiring plasma. In former times fluoride-oxalate was the anticoagulant of choice for blood glucose determination, because it halts glycolysis by the red cells. However, fluoride may interfere with certain chemical procedures (specifically the glucose oxidase method for blood glucose determination) and should be used only for blood lactate -determination or in selected circumstances in which glucose determinations are required and samples must be held for some period of time without refrigeration. Citrate is the anticoagulant of choice for clotting tests and blood typing. Ethylenediaminetetraacetic acid (EDTA) is the anticoagulant most often used for hematologic evaluation. Both citrate and EDTA are chelating agents, which may interfere with a wide variety of chemical determinations.

Table 22-4 Recommended Anticoagulants for Hematologic or Clinical Chemistry Evaluation

PT, Prothrombin time; PTT, partial thromboplastin time; T₃, triiodothyronine; T₄, thyroxine.

Modified from Brobst DF, Parry BW: Normal clinical pathology data. In Robinson NE, ed: Current therapy in equine medicine, ed 2, Philadelphia, 1987, Saunders.

Samples should be submitted as soon after collection as possible, but circumstances may require storage of some samples for 12 to 24 hours. For collection of serum, whole blood should be allowed to clot before refrigeration. Serum should be separated from the red cells immediately after clot formation and then kept refrigerated. Samples should be stored in clean containers free from exposure to sunlight, medications, or chemicals. If whole blood is left at room temperature for longer than 60 minutes, blood glucose will be falsely low as a result of red cell glycolysis. Storage of whole blood may result in in vitro hemolysis, with the potential for misleading increases in the serum or plasma enzymes AST and lactate dehydrogenase (LDH) due to hemolysis. Failure to separate serum or plasma from the red cells within an hour of collection may lead to leakage of erythrocyte potassium and a falsely elevated serum or plasma potassium concentration.

Stress, transportation, excitement, and handling produce physiologic responses in animals that affect a variety of hematologic and biochemical parameters. This is most evident in the horse, which shows marked increases in red cell mass and to a lesser extent plasma protein concentration in response to excitement, exercise, or catecholamine administration. The red cell count and hemoglobin concentration can increase by as much as 50%, whereas plasma protein concentration may increase by 1 to 2 g/dL. Leukocytosis is induced as the marginating leukocyte pool is mobilized into the general circulation. Prolonged stress results in the release of endogenous corticosteroids, which produce the typical “stress response” in the leukogram. A similar leukogram is found in race horses some 4 to 6 hours after racing. The combination of catecholamine and glucocorticoid release associated with stress, transport, and excitement, as well as with many gastrointestinal catastrophes, may result in markedly elevated blood glucose concentrations (up to 400 mg/dL). Modest elevations (twofold to fourfold increase) in muscle-derived enzymes occur in association with prolonged transport or endurance exercise.

Large losses or compartmentalization of sodium-containing fluid accompanies many systemic disorders, particularly digestive problems such as diarrhea, colic, displacement of viscera, excessive sweat losses, and some urinary tract diseases. These forms of dehydration lead to decreases in plasma volume, which are indicated by moderate-to-marked increases in the PCV and TPP concentration. The concentration of other compounds dissolved in the plasma may also increase as a result of decreases in the plasma volume. The concentrations of compounds that are largely protein-bound, such as calcium, are generally closely related to protein concentration. More than 50% of serum calcium is bound to albumin. Increases or decreases in plasma protein concentration normally result in proportional changes in total serum calcium concentration, whereas the physiologically active ionized calcium may remain unchanged.

Diseases that cause a reduction in effective circulating fluid volume often also cause alterations in renal function. This so-called “prerenal azotemia” results in moderate to marked elevation in BUN and creatinine. Although this is generally considered primarily a prerenal azotemia, real pathologic changes in the kidneys often are associated with the systemic processes initiated by these disorders. Reevaluation of renal function (urinalysis BUN, creatinine) in these patients during the course of disease is important because it affects prognosis, response to fluid administration, and the potential for nephrotoxicity and systemic toxicity of a variety of chemotherapeutic agents.

Fasting laboratory data are important for evaluation of many disease conditions in human and small animal patients. Truly fasting conditions are rather difficult given the large and complex gastrointestinal tract of most herbivores and are thus seldom used. The feeding of animals in relation to sample collection can, however, have an impact on the data obtained. Hay feeding in horses is reported to affect sodium, potassium, and protein concentrations within the first few hours after feeding. Animals feeding on lush green pasture or large amounts of silage may have slightly different parameters from those fed high-concentrate rations. The anion-cation balance of the ration has an impact on relative serum electrolyte concentration, acid-base balance, and urine pH and urinary electrolyte excretion. Lactescent (cloudy) plasma may be observed in samples from nursing foals or calves. The fluid intake of the normal nursing neonate may range from 100 mL/kg/day to more than 250 mL/kg/day. This high fluid intake is reflected by a commensurately high output of urine with a low specific gravity and low osmolar content.

The administration of certain medications may have an impact on some laboratory parameters. Tranquilization may be necessary for restraint and safe sample collection. The practitioner should be aware that tranquilizers often decrease red cell mass and plasma protein concentration. This is particularly true of the phenothiazine-derivative tranquilizers when used in the horse. Xylazine administered to large animals produces a modest catecholamine release, which may be evidenced by the slight sweating response seen in many horses sedated with this drug. Glucose concentration will increase modestly in response to the xylazine-induced catecholamine release. Repeated intramuscular injections with certain antibiotics (especially erythromycin and tetracycline) or other preparations that are locally irritating may produce slight-to-moderate elevations in muscle-derived serum enzyme activities. Intravenous administration of certain drugs and compounds such as dimethyl sulfoxide (DMSO) can produce intravascular hemolysis and hematuria. The amount of hemolysis in these circumstances is relatively small and of little consequence, except that it can cause confusion as to why hemoglobinuria occurred.

Packed Cell Volume and Total Plasma Protein

Changes in the plasma volume generally are reflected by changes in the PCV and the TPP concentration. In dehydrated humans, changes in the PCV are believed to be the more reliable guide to changes in plasma volume because substantial protein fluxes into and out of the circulation have been shown to occur. However, in most animal species the range of normal for the PCV is much wider than for the TPP concentration. This is particularly true of horses, in which excitement, pain, or catecholamine release can produce variable mobilization of splenic erythrocytes, making it difficult to obtain a truly resting PCV. For these reasons, precise quantitative estimation of a change in plasma volume using these parameters is more complex and less reliable in large animal species. As plasma volume increases or decreases, the change in the PCV is always less than the change in the TPP concentration. However, a large disparity in the changes in the PCV and the TPP concentration in a patient with a history of loss of sodium-containing fluid and clinical evidence of reduced effective circulating fluid volume suggests blood or protein loss. Marked increases in the PCV with a normal-to-low TPP concentration frequently are encountered in animals with acute protein-losing enteropathies such as salmonellosis or equine toxic enteritis. In horses undergoing treatment for diarrhea, the excessive administration and retention of sodium-containing fluids is a key factor in the development of edema and hypoproteinemia. Blood loss generally results in a decrease in both PCV and TPP concentration.

Serum Sodium

The serum sodium concentration is a function of the exchangeable cation content (i.e., the exchangeable sodium [Na] in the extracellular fluid [ECF] volume plus the exchangeable potassium [K] in the intracellular fluid [ICF] volume relative to total body water), as indicated in the following formula:

Changes in the sodium concentration reflect the net changes in this relationship and often do not represent accurately the changes in sodium balance. Changes in water balance are thus primarily responsible for changes in the serum sodium concentration. Hyponatremia is an indication of a relative water excess, whereas hypernatremia is an indication of a relative water deficit.

Dehydration is defined as a loss of body water (fluid volume contraction). It occurs in a variety of clinical circumstances. The serum sodium concentration provides a means of categorizing dehydration in a physiologically meaningful way. Hypertonic dehydration, which occurs when water losses exceed the losses of sodium and potassium, is indicated by hypernatremia. The response of horses to feed and water deprivation is an example of this form of dehydration. Isotonic dehydration occurs with a balanced loss of water and electrolytes—that is, approximately 140 to 150 mEq of sodium plus potassium (Na + K) for each liter of water lost. Because the relative water balance has not changed, the serum sodium concentration remains unchanged despite the accumulation of what may have been a substantial sodium deficit. The early stages of acute diarrhea and the dehydration of heavily sweating endurance horses are examples of isotonic dehydration. Hypotonic dehydration occurs when the losses of exchangeable cations (Na + K) exceed the net change in water balance; this condition is indicated by hyponatremia. Hypotonic dehydration often is seen in animals with subacute or chronic diarrhea that develop substantial water and electrolyte deficits but then replace part of the water deficit through water consumption. Fig. 22-1 shows the compartmental distribution of fluid between the ECF volume and the intracellular fluid (ICF) volume in four situations.

Fig. 22-1 Compartmental distribution of fluid between the extracellular fluid (ECF) volume and the intracellular fluid (ICF) volume in a 450-kg horse with normal fluid balance; with isotonic fluid volume contraction; with hypotonic fluid volume expansion; and with hypertonic fluid volume contraction.

Modified from Kaneko JJ, Harvey JW, Bruss ML, eds: Clinical biochemistry of domestic animals, ed 5, New York, 1977, Academic.

Serum Potassium

The serum potassium concentration is influenced by factors that alter internal balance (the distribution of potassium -between the ECF and the ICF) and those that change external balance (potassium intake and output). Changes in the serum potassium concentration occur in a wide variety of clinical circumstances and have profound neuromuscular effects that are largely the result of changes in cell membrane potential. The responses to dehydration and acid-base imbalance often complicate the evaluation of the potassium concentration. For example, calves with acute diarrhea often develop potassium depletion because of excessive losses and inadequate intake, but the serum potassium concentration of these animals usually is normal to increased as the result of renal shutdown and the metabolic acidosis induced by dehydration, sodium depletion, and hypovolemia. Hypokalemia may become evident only as other fluid and electrolyte losses are replaced.

Measuring the erythrocyte potassium concentration is relatively easy and has been suggested as an aid in assessing the need for potassium supplementation in racehorses with recurrent muscle disease. However, experimental studies in horses indicate that the erythrocyte potassium concentration does not always accurately reflect potassium deficits.

Serum Chloride

Alterations in the chloride concentration usually are associated with nearly proportional changes in the sodium concentration as the result of changes in relative water balance. In addition, the chloride concentration tends to vary inversely with the bicarbonate concentration; therefore, when disproportionate changes in the chloride concentration relative to sodium occur, significant acid-base imbalances should be anticipated. Disproportionate increases in chloride are associated with a normal-to-low anion gap hyperchloremic metabolic acidosis, but they also are seen as a result of the compensating responses for a primary respiratory alkalosis (Box 22-5). A striking hyperchloremic metabolic acidosis has been reported in horses with renal tubular acidosis.^9,10 Disproportionate decreases in chloride relative to sodium characteristically are seen in metabolic alkalosis but also may be seen as part of the compensating response for chronic primary respiratory acidosis (Box 22-6). Hypochloremic metabolic alkalosis is a common feature in many digestive disorders of ruminants and is caused by loss of chloride-rich fluids or sequestration of such fluids in the abomasum and forestomachs.

Osmolality

Measurement of the serum osmolality provides an indication of relative water balance in much the same way as the serum sodium concentration does. In most circumstances these parameters are closely correlated. Comparing the measured osmolality with the calculated osmolality, as determined from the measured concentrations of the major solutes in serum (sodium, glucose, and urea), provides a means of determining if the serum water content deviates widely from normal or if foreign, low—molecular-weight substances are present in the blood. The difference between the measured osmolality and the calculated osmolality is called the osmolar gap. Decreases or increases in the osmolar gap could indicate laboratory error, but increases of more than 10 mOsm/kg generally are the result either of a decrease in the serum water content (caused by hyperlipidemia or hyperproteinemia) or of the presence of abnormally high concentrations of low—molecular-weight substances in the serum. These substances can include a variety of exogenous and potentially toxic compounds such as mannitol, ethanol, methanol, propylene glycol, ethylene glycol, isopropanol, ethyl ether, acetone, trichloroethane, and paraldehyde.

Serum Calcium

Calcium plays a vital role in many of life’s processes, including maintenance of neuromuscular excitability, permeability of cell membranes, conduction of nerve impulses, muscle contraction, and blood clotting. For these reasons the serum calcium concentration or, more correctly, the ionized calcium concentration normally is maintained within a relatively narrow range, despite wide variation in intake and output. Calcium metabolism is regulated by dietary factors, vitamin D and its active metabolites, and the hormones parathormone and calcitonin. The serum calcium concentration is maintained by adjusting intestinal absorption, renal excretion, and mobilization of available calcium from the large stores in bone. Calcium exists in the serum in three forms: ionized calcium, complexed calcium, and protein-bound calcium. Ionized calcium, which normally constitutes 40% to 60% of the total calcium, is the physiologically active form of calcium. Protein binding (protein-bound calcium normally constitutes 40% to 50% of the total calcium) can cause confusion. Hyperalbuminemia may result in a modest hypercalcemia, whereas hypoproteinemia, especially hypoalbuminemia, regularly results in a moderate hypocalcemia. The ionized calcium concentration generally remains within normal limits, despite increases or decreases in total calcium associated with the change in protein concentration. The acid-base balance has additional influence on the amount of ionized and protein-bound calcium. Alkalosis reduces ionized calcium and increases protein binding, whereas acidosis produces the opposite effect. Ion-specific electrodes are available for determining the ionized calcium level, which can be very useful if blood samples are handled appropriately. Most diagnostic laboratories provide the total serum calcium value, which is composed of ionized, complexed, and protein-bound calcium.

Serum Phosphorus

Phosphorus is found primarily in the skeleton and teeth in close association with calcium in the intricate and dynamic crystalline structure of bone. Intracellularly, phosphate plays an essential role in the degradation and synthesis of many compounds. Also, as adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP), it is the primary form of energy storage and transfer required for almost all of life’s processes. Phosphorus in the ECF exists primarily as the buffer pair H₂PO⁻³ and HPO⁻⁴ and plays a role in acid-base balance. Like calcium, phosphorus is regulated by dietary factors, the active metabolites of vitamin D, and the hormones parathormone and calcitonin. Imbalances of calcium and phosphorus or the presence of compounds that bind these substances in the gut can produce serious imbalances that are not always evident on analysis of serum samples. Measurement of urinary output or creatinine clearance ratios for calcium and phosphate are simple and helpful procedures. They provide an indication of an imbalance while more definitive procedures such as ration analysis are contemplated.

Serum Magnesium

Disturbances of magnesium metabolism occur principally in cattle and sheep. Complex nutritional and environmental interactions contribute to a variety of clinical syndromes attributed to magnesium deficiency and the onset of tetany in grazing animals.

Metabolic Acidosis

Metabolic acidosis is characterized by a decrease in pH and bicarbonate concentration. Metabolic acidosis is traditionally thought to be produced by the addition of hydrogen ions or the loss of bicarbonate ions. The most common causes include the lactic acidosis of ruminal overload, hypovolemia associated with loss or compartmentalization of sodium-containing fluid, ketoacidosis in ketosis and pregnancy toxemia, loss of bicarbonate-rich saliva with oral diseases or esophagostomy in cattle, gastrointestinal loss of bicarbonate as a result of diarrhea, and renal failure, which may result in decreased ability to excrete hydrogen and thus to retain bicarbonate (Box 22-13). Other causes include ingestion of certain drugs or toxic compounds such as salicylate, methanol, ethylene glycol, or paraldehyde. Increased ventilation provides the compensating respiratory response for metabolic acidosis, and a decline in the PCO₂ generally begins within minutes. This temporarily minimizes the fall in blood pH, but long-term correction of metabolic acidosis requires renal bicarbonate retention and enhanced acid excretion. Complete correction of metabolic acidosis may be difficult in patients with intrinsic renal disease or those with diseases such as renal tubular acidosis that impair the ability of the kidneys to excrete acid or retain bicarbonate, or both.

Metabolic Alkalosis

Metabolic alkalosis is characterized by an increase in pH and bicarbonate. It occurs fairly commonly in domestic animals, particularly in association with digestive disturbances in ruminants. An initiating process capable of generating alkalosis is necessary and must be coupled with additional factors to maintain metabolic alkalosis. Generation of metabolic alkalosis is traditionally thought to be caused by excessive hydrogen loss, bicarbonate retention, or contrac-tion alkalosis (Box 22-14). Contraction alkalosis occurs when the ECF volume is reduced because of loss or sequestration of fluids high in sodium and chloride but without proportionate loss of bicarbonate. This is a contributing mechanism for the generation of the metabolic alkalosis reported in heavily sweating endurance horses and in response to the diuretic furosemide in the horse. The most common causes of increased hydrogen ion loss are the gastrointestinal losses caused by salivary secretions in ponies after surgical esophagostomy¹⁵; massive gastric reflux associated with anterior enteritis, ileus, or small bowel obstruction in horses; and sequestration of fluid in the abomasum and forestomach associated with a variety of gastrointestinal displacements or functional disturbances (vagal indigestion) of ruminants. Continuous salivary losses in horses after surgical esophagostomy result in transient metabolic acidosis followed by progressive metabolic alkalosis. Most of these disorders cause significant dehydration and sodium, chloride, and potassium deficits.

The factors responsible for maintaining metabolic alkalosis involve impaired renal bicarbonate excretion. These factors are associated with the renal response to decreases in the effective circulating fluid volume, chloride depletion, or potassium depletion. Renal tubular sodium resorption is enhanced in response to hypovolemia. Maintenance of electroneutrality requires that sodium resorption in the proximal tubule be accompanied by a resorbable anion, whereas in the distal tubule, sodium resorption is associated with the secretion of another cation, usually hydrogen or, to a lesser extent, potassium. Chloride is the only resorbable anion normally present in appreciable quantities in the proximal tubular fluid. In metabolic alkalosis, plasma bicarbonate is increased and the chloride concentration is generally decreased as the result of disproportionate chloride losses. The relative lack of the resorbable anion chloride in the proximal tubule thus allows a larger amount of sodium to reach the distal tubule, where aldosterone and other factors enhance hydrogen or potassium loss into the tubular lumen in exchange for sodium. Potassium depletion reduces or eliminates potassium exchange as a means of sodium retention, thus placing greater emphasis on hydrogen ion exchange. Because renal hydrogen excretion is linked with bicarbonate resorption, the excess bicarbonate cannot be eliminated, and metabolic alkalosis is maintained. This is the reason for the paradoxic aciduria seen in some patients with metabolic alkalosis, and it is the reason these patients respond when given intravenous fluids containing chloride -and potassium. The compensating respiratory response to metabolic alkalosis is hypoventilation, resulting in an increase in the PCO₂. Excessive bicarbonate administration is an additional potential cause of metabolic alkalosis. Most normal animals can tolerate large doses of bicarbonate, and excesses are rapidly eliminated by renal excretion.¹⁶ However, patients with decreases in effective circulating fluid volume, particularly when coupled with potassium or chloride deficits, may not tolerate a bicarbonate load because renal clearance of excess bicarbonate is likely to be impaired. Attempts to alter the acid-base balance, and thereby affect the athletic performance of racing horses, by prerace administration of high doses of sodium bicarbonate—containing “milk shakes” has become a major concern around the world. This has stimulated substantial research on the acid-base balance of horses before and after racing. In many racing jurisdictions stringent prerace or postrace standards for blood pH or bicarbonate levels (or both) have been enacted to control this practice. The prerace administration of the diuretic furosemide is allowed in some racing jurisdictions. This drug results in a mild metabolic alkalosis with increased bicarbonate, and allowances for this may be included in some regulations. Racing supplements (which may contain sodium bicarbonate), bicarbonate precursors, and diets that supply a high cation-anion balance also have the potential to produce a significant metabolic alkalosis.

Respiratory Acidosis

Respiratory acidosis is characterized by a decrease in pH and an increase in PCO₂, which develop because of decreased effective alveolar ventilation. CO₂ diffuses through the lungs much more readily than oxygen; thus diseases that compromise ventilation normally result in decreases in Po₂ before significant increases in PCO₂ develop. The most common causes are acute upper respiratory obstruction and primary pulmonary diseases, including pneumonia, pneumothorax, and chronic obstructive lung disease (Box 22-15). Diseases -or drugs that affect the respiratory center of the central nervous system also may produce respiratory acidosis, as can general anesthesia. The compensating response for respiratory acidosis is renal retention of bicarbonate. This response requires days to develop and is seen only in chronic respiratory acidosis. Exogenous bicarbonate does not correct respiratory acidosis and should not be administered to affected patients.

Respiratory Alkalosis

Respiratory alkalosis is caused by hyperventilation, which may be stimulated by hypoxemia associated with pulmonary disease, congestive heart failure, severe anemia, or neurologic disorders (Box 22-16). The initial compensating response to acute respiratory alkalosis is a modest decline in the ECF bicarbonate concentration, the result of cellular buffering. Subsequent renal responses result in a decrease in the ECF bicarbonate concentration through reduced renal bicarbonate resorption. The decline in bicarbonate may be offset by chloride retention; thus hyperchloremia and decreased PCO₂ may be associated with compensated respiratory alkalosis, as well as with compensated metabolic acidosis. Compensating responses for chronic respiratory alkalosis that lasts several weeks actually may be sufficient to return pH to normal.

Mixed Acid-Base Imbalances

Mixed acid-base disorders occur when several primary acid-base imbalances coexist.¹⁷ Metabolic acidosis and alkalosis can coexist, and either or sometimes both may occur with either respiratory acidosis or respiratory alkalosis. The following factors should be considered when evaluating possible mixed acid-base disorders:

Mixed acid-base abnormalities occur with some frequency in domestic animals and often are overlooked. The practitioner must be aware of the potential for mixed acid-base imbalances to correctly interpret blood gas data in complex clinical situations.

Anion Gap

The anion gap can be an extremely helpful tool for categorizing causal factors in acid-base imbalances and may prove a useful prognostic guide in animals with serious digestive disorders. The anion gap can be calculated as the difference between the major cations (sodium plus potassium) and the measured anions (chloride plus bicarbonate). The anion gap normally is 12 to 16 and provides an approximation of the so-called “unmeasured anions.” These are anions that are not measured routinely in the clinical laboratory; they include the anionic equivalents of plasma proteins (particularly albumin), sulfate, phosphate, lactate, ketones, and a variety of inorganic anions. Significant differences exist in the normal range of the anion gap among species, and there also may be age-related differences. Foals are reported to have a larger anion gap than adult horses.

Hypoalbuminemia and hyperchloremic metabolic acidosis are the most common causes of a decrease in the anion gap resulting from decreases in unmeasured anions. The cause of normal to low anion gap hyperchloremic metabolic acidosis often can be differentiated on the basis of the serum potassium concentration. Animals with hyperchloremic metabolic acidosis associated with gastrointestinal fluid losses or renal tubular acidosis most often manifest hypokalemia, whereas hyperkalemia generally is seen in patients with decreased mineralocorticoid secretion (Addison’s disease) or renal failure with renal shutdown. Decreases in the anion gap can be seen with increases in cationic proteins associated with polyclonal gammopathy or multiple myeloma. Decreases in the anion gap also result from overhydration caused by decreases in the protein concentration and changes in the relative concentration of plasma sodium and chloride.

Most commonly, high anion gap acidosis is associated with accumulation of a metabolizable acid such as lactic acid associated with anaerobic exercise, grain overload, or hypovolemic shock. The commonly employed laboratory procedures for the determination of lactate only measure the L form of lactate. Microbial fermentation in the gastrointestinal tract may result in the production of both the D and L forms of lactate. It has recently been shown in milk-fed calves and kids that accumulation of D-lactate is a major factor in the profound acidosis associated with certain digestive disorders. Unfortunately, at present, determination of D-lactate requires special procedures that may not be readily available. Ketoacidosis, uremic acidosis, and poisoning with a variety of anionic poisons result in increases in nonmetabolizable acids that are also causes of an increased anion gap. When a high anion gap metabolic acidosis is found, a thorough search for the potential causes of the accumulated unmeasured anions is indicated. The anion gap also is useful for identifying mixed acid-base imbalances. A mixed metabolic acid-base imbalance should be suspected when the change in the anion gap does not approximate the change in bicarbonate. Increases in the anion gap can be associated with dehydration and contraction alkalosis caused by changes in the protein concentration and the relative concentration of plasma sodium and chloride.

Bicarbonate and Total Carbon Dioxide

Bicarbonate accounts for approximately 95% of the measured CO₂; thus the total CO₂ (TCO₂) or “CO₂ content” of serum or plasma provides a measure of metabolic changes in the acid-base balance. Most automated chemistry profiles now provide the bicarbonate level directly, whereas some may still provide the TCO₂. Bicarbonate or TCO₂ is decreased in metabolic acidosis and increased in metabolic alkalosis. However, the bicarbonate or TCO₂ values provide only a crude indication of acid-base status. When acid-base abnormalities are suspected, appropriate samples should be submitted for blood gas determination.

Buffer Base, Standard Bicarbonate, and Base Excess or Base Deficit

The terms buffer base, standard bicarbonate, and base excess (or base deficit) represent derived calculated estimates of the metabolic component of acid-base balance. The buffer base indicates the sum of all the buffer anions in blood under standardized conditions. The standard bicarbonate is the plasma bicarbonate concentration that would be found under specific conditions that eliminate respiratory influences on the values obtained. The base excess or base deficit often is supplied in routine assessment of acid-base balance; it indicates the deviation of bicarbonate from normal. The calculated base deficit provides a means of estimating the amount of bicarbonate required to correct metabolic acidosis. The bicarbonate estimate is calculated by multiplying the base deficit by the probable bicarbonate space (about 40% to 60% of body weight), as in the following equation:

Sorbitol Dehydrogenase

SDH is a liver-specific enzyme in all large animal species, and increases in this enzyme indicate hepatocellular damage and leakage of enzymes. Increases in SDH also are seen with obstructive or strangulating gastrointestinal lesions and with acute toxic enteritis as a result of liver damage associated with absorption of bacteria or their toxins (or both) from the damaged bowel into the portal circulation. This enzyme is a sensitive indicator of liver damage, and modest increases may be seen with anoxia, acute anemia, or general anesthesia. The half-life of SDH in the circulation is short (a matter of hours), and elevations indicate active and ongoing liver damage. This enzyme is not stable when stored at room temperature, but refrigerated samples may yield useful results after several days of storage.

Creatine Kinase

CK is a highly sensitive and specific indicator of muscle damage in domestic animals. Although CK is found in both cardiac and skeletal muscle, elevations of this enzyme most commonly are associated with exertional myopathies (rhabdomyolysis) and also are seen as musculoskeletal manifestations of systemic disease. Intramuscular injections, vigorous exercise, or prolonged shipping may result in modest releases (up to a fourfold increase over resting values) of CK into the circulation without producing histologic evidence of muscle damage. Endurance exercise may lead to moderate elevation of CK (2000 to 15,000 IU/L) in some horses that show no recognizable sign of exertional myopathy. The half-life of this enzyme in the circulation is very short (2 hours in horses and 4 hours in cattle), and even marked elevations in CK may return to normal within 12 to 24 hours after a single muscle insult. Although marked elevation of CK can be a guide to the extent of muscle damage, the short half-life and the potential for continuing myonecrosis have a marked influence on the enzyme activity observed at any point in time. A persistent elevation of CK suggests a process resulting in active and continuing muscle damage and provides grounds for resting athletic horses. Elevated CK provides no information on the factors responsible for the rhabdomyolysis. Hemolysis may produce falsely high values for CK.

Aspartate Aminotransferase

AST is found in high concentration in a variety of tissues, including skeletal and cardiac muscles, the erythrocytes and kidneys, and the liver. This enzyme is a nonspecific indicator of tissue damage and tends to be less sensitive to mild insults than the tissue-specific enzymes SDH and CK. The half-life of AST in the circulation is relatively long, and elevations may persist for as long as 10 days after an episode of myonecrosis or liver damage. As a general rule, extensive muscle necrosis tends to produce much higher elevations of AST than severe liver necrosis. This enzyme is most useful when compared with the tissue-specific enzymes as determined sequentially over the time course of a disease process. Elevations of CK and AST indicate muscle damage, whereas elevations of SDH and AST indicate liver damage. Marked but transient elevations of CK and SDH are associated with a single insult to the muscles and liver, respectively, whereas AST increases gradually and remains elevated for a much longer time. Thus a moderate to marked increase in AST in an animal with progressively declining SDH or CK indicates that some tissue damage occurred within the past 7 to 10 days but also that the process may no longer be active. This often is a favorable prognostic indicator. Persistent elevation of or a progressive increase in CK or SDH and AST over time indicates an active, continuing process of tissue damage, and the prognosis is more guarded. AST is relatively stable at room temperature, but hemolysis or lipemia may interfere with the assay.

γ-Glutamyltransferase

GGT is an important marker of hepatobiliary disorders and cholestasis in large animals. GGT is quite stable, and reliable results can be obtained from samples submitted several days after blood samples have been drawn, provided the serum is refrigerated. The activity of this enzyme is highest in the cells of the periportal region of the liver, in the pancreas, and in the renal tubular cells. Pancreatic diseases resulting in inflammation and necrosis are relatively rare in large animal species. Damage to the renal tubular cells leads to a release of GGT into the tubular lumen and the urine. Because this enzyme is a relatively large molecule, it is not resorbed into the systemic circulation, and renal tubular damage does not result in elevated serum GGT activity. Increases in GGT relative to creatinine in the urine have been used as an index of acute renal tubular damage. However, the validity of the normal range for this ratio in horses has been questioned.

In large animal species an elevation in serum GGT is one of the more reliable indicators of damage to the liver and biliary obstruction. Disease processes such as pyrrolizidine alkaloid intoxication, chronic active hepatitis, cholangiohepatitis, and cholelithiasis produce liver damage, primarily in the periportal region, leading to marked and persistent elevation of GGT activity in the serum. In these instances, elevations in serum ALP activity generally are associated with the increase in GGT. Two syndromes, fatty liver syndrome in dairy cows and hyperlipemia syndrome of periparturient mares of pony and miniature horse breeds, are associated with liver damage, which is often reflected by elevation of GGT.

Most suckling neonatal large animals have high levels of GGT activity in their serum. This is the result of absorption of maternal GGT, which is present in relatively high levels in the colostrum. Elevation of GGT in neonates should be regarded as a normal finding unless it is associated with other evidence of liver disease. The normal range of serum GGT activity for burros, donkeys, and asses may be substantially higher (two to three times) than the normal range of serum GGT activity for horses. Caution should therefore be used when evaluating this enzyme in these species. Elevation in serum GGT activity has been reported in thoroughbred racehorses that are performing below expectations. The reasons for the increase in GGT are not known, but horses often respond to a period of rest or reduction in workload. These horses show little histologic evidence of liver damage, and other indices of liver damage and dysfunction usually are within normal limits. The stress of training may be associated with an elevated GGT. Certain trainers, often highly successful trainers, appear to have a disproportionately large number of horses with elevations of this enzyme. The normal range for GGT of thoroughbreds in race training may be slightly higher than that of normal sedentary horses.

Alkaline Phosphatase

ALP is used in most species as a marker for intrahepatic or extrahepatic obstruction of the biliary system. The enzyme is also released by osteoblasts from metabolically active bone. This may be the reason that young, rapidly growing animals normally have high levels of serum ALP. Elevations in ALP are also reported in cases of rickets and healing fractures. The intestinal isoenzyme of ALP is very similar to the ALP isoenzyme found in neutrophils. Elevations in the ALP activity of abdominal fluid in horses with intraabdominal disease may reflect the release of this enzyme from the neutrophils rather than being a specific marker of damage to the bowel.

ALP has been useful for evaluating liver disease in large animals, particularly in horses with pyrrolizidine alkaloid intoxication, chronic active hepatitis, and cholangiohepatitis, and in some patients with cholelithiasis. A profound elevation in ALP activity is thought to reflect periportal liver damage and biliary obstruction in these patients. A moderate to marked elevation in ALP may be observed in a wide range of disorders resulting in hepatic necrosis and intrahepatic cholestasis. Because this enzyme is not organ specific in large animals, elevations in ALP activity must be interpreted in relation to more organ-specific enzymes such as SDH and GGT.

Other Enzymes

Lactate dehydrogenase is found in relatively high concentrations in a variety of organs and tissues of the body from the heart, liver, muscle, and kidney to the erythrocytes and leukocytes. An elevation in serum LDH enzyme activity must be evaluated in relation to other, more organ-specific enzymes. LDH isoenzyme analysis can be helpful in differentiating organ system damage, but the analysis is time-consuming and not always available. An elevation in LDH activity is expected in hepatic necrosis and serves as an indicator of an active disease process. Extensive muscle damage and rhabdomyolysis tend to result in a more massive release of enzyme and much higher serum enzyme activity. A modest elevation in LDH may be seen in some hemolytic disorders and some cases of leukemia. Blood samples must be handled with care, because hemolysis results in falsely elevated serum LDH activity.

Glutamic dehydrogenase and ornithine carbamoyltransferase are two enzymes that are reported to be sensitive indicators of hepatic necrosis in ruminants. Alanine aminotransferase (ALT) is an important liver-specific enzyme that has wide application in small animals and is often included in automated chemistry profiles. This enzyme has not been useful for evaluation of liver disease in large animal species, and occasionally horses with marked rhabdomyolysis and no other evidence of liver disease show an elevation in serum ALT activity.

Hypoglycemia

Fasting usually does not result in hypoglycemia except in neonatal animals (Box 22-19). Newborns have limited energy reserves, and any disease, injury, congenital defect, maternal rejection, agalactia, or management error that limits energy intake can result in marked hypoglycemia associated with profound depression, even coma. Rapid, semiquantitive field tests for blood glucose (Dextrostix* and Chemstrip BG^†) provide a practical means of early recognition of this problem. Hypoglycemia may be seen in animals with acute toxic enteritis, coliform mastitis, septicemia, and colic associated with strangulated bowel, as well as in the later stages of endotoxemia and in some horses with exhaustion after prolonged exercise. Hypoglycemia is a reasonably consistent feature with primary ketosis and fat cow syndrome in cattle, with pregnancy toxemia in sheep and goats, and in hyperlipemia syndrome, which is seen primarily in pregnant or lactating ponies.

Hyperglycemia

Hyperglycemia may be seen with excitement, transportation, or stress and probably is mediated by increases in catecholamine and glucocorticoid hormones (Box 22-20). The stress and pain of acute severe colic in horses frequently results in hyperglycemia, and elevations of blood glucose above 250 mg/dL are associated with a poor prognosis in such cases. Endotoxemia initially results in a transient hyperglycemia that may be followed by marked hypoglycemia in the terminal stages of the toxemia. The later stages of Cushing’s syndrome in horses generally are associated with a non—insulin-responsive hyperglycemia and glycosuria. Similar changes can be induced transiently when exogenous glucocorticoid hormones are administered at a high dose rate. Insulin-responsive diabetes mellitus rarely occurs in large animals but has been reported in association with destructive pancreatic lesions.

Specific Gravity

Specific gravity is defined as the weight of urine relative to the weight of distilled water. The specific gravity of serum is approximately 1.008 to 1.012. The urine specific gravity is an indicator of renal function, because it reflects the action of the renal tubules and collecting ducts on the glomerular filtrate by providing an estimation of the number of particles dissolved in the urine. It is usually measured by refractometry. Urine dipsticks with reagent pads for estimating the urine specific gravity in humans should not be used for this purpose in domestic animals, because a poor correlation has been reported compared with refractometry in large animals.²⁶ Normal animals have the capacity to dilute their urine specific gravity to less than 1.010 and to concentrate to greater than 1.050. The normal range reported for most adult large animals is 1.020 to 1.050. Suckling neonatal animals normally produce a very dilute urine with a specific gravity below 1.010. Although this could represent immature renal function, it more likely reflects their high-volume fluid intake, because normal neonates can concentrate their urine if fluids are withheld.

Failure to produce a concentrated urine, as reflected by a specific gravity below 1.020, in the face of dehydration is an indication of altered renal function, which may be caused by primary renal disease, diabetes insipidus, medullary washout, or nephrogenic diabetes insipidus. With severe and chronic sodium depletion, tubular sodium may be insufficient to sustain normal countercurrent mechanisms for water resorption. This process is called medullary washout and has been reported as a cause of polyuria in lactating dairy cattle on a salt-deficient diet. At least 50% of renal function must remain for highly concentrated urine to be produced. Isosthenuria, a urine specific gravity that remains around 1.010 despite variation in hydration, occurs when renal disease progresses to the point at which renal function is reduced to less than a third of normal. Altered renal function in these animals is usually reflected by a moderate to marked elevation in BUN and creatinine levels and by other changes in the urinalysis. Hyposthenuria occurs when the specific gravity remains below 1.010; this indicates altered release of or response to antidiuretic hormone, as occurs with diabetes insipidus, medullary washout with chronic sodium depletion, nephrogenic diabetes insipidus, psychogenic polydipsia, and chronic liver failure in some horses.

pH

The normal range of urine pH for adult herbivores is alkaline, with values ranging from 7 to 9. Neonatal foals tend to have a slightly acid urine with a pH below 7. Aciduria in adults may be seen in postrace samples collected from racehorses, after prolonged fasting, with ketosis in ruminants, or in response to metabolic acidosis. A paradoxic aciduria frequently is seen in ruminants with hypochloremic, hypokalemic metabolic alkalosis, as was explained earlier. Urine pH is also influenced by the cation-anion balance of the diet.

Protein

Protein normally is not detected in urine, although a false-positive protein reaction may be noted on urine dipsticks if the sample is strongly alkaline. Urine with a positive reaction for protein on dipsticks should be checked for protein by a chemical method. Proteinuria should be evaluated in relation to the other findings in the urinalysis. Persistent and strongly positive reactions for protein in the absence of leukocytes, red cells, bacteria, or casts suggest glomerular protein loss, as in glomerulonephritis or amyloidosis. The presence of bacteria and leukocytes with proteinuria suggests sepsis in the urinary tract, whereas hemorrhage or inflammation in the urogenital tract often is associated with proteinuria.

Glucose

Glucose is not found in the urine of normal large domestic animals unless the blood glucose level increases above the renal threshold, which is thought to be approximately 100 to 140 mg/dL in ruminants and approximately 160 to 180 mg/dL in horses. The causes of hyperglycemia and glycosuria were described in a previous section; they include Cushing’s syndrome, stress, and catecholamine or glucocorticoid hormone release. Hyperglycemia and glycosuria can be created iatrogenically when glucose-containing fluids are administered at an excessive rate. Glycosuria is a fairly consistent finding in sheep with enterotoxemia type D (pulpy kidney). The presence of glycosuria without hyperglycemia strongly suggests renal tubular damage resulting from a toxic or ischemic insult.

Occult Blood

Both myoglobin and hemoglobin give a positive reaction on urine occult blood dipsticks. In most instances proteinuria shows a positive result as well. A tentative clinical impression sometimes can be formed to differentiate myoglobin from hemoglobin in dark urine if the sample is shaken vigorously in a closed, transparent container. Myoglobin tends to produce a brown foam, whereas hemoglobin produces a reddish foam. Hemoglobinuria caused by intravascular hemolysis is generally associated with clinical and hematologic evidence of a hemolytic anemia. Hematuria may result in a positive occult blood reaction if lysis of some of the intact red blood cells has occurred. This is likely to happen if the urine is very dilute or if it is held at room temperature for an extended period before analysis. False-positive reactions can occur if microbial peroxidase or oxidizing contaminants are present.

Myoglobin

Myoglobinuria should be associated with clear clinical and clinicopathologic evidence of extensive muscle damage. The ammonium sulfate precipitation method of differentiating hemoglobin from myoglobin is imprecise and frequently fails to detect myoglobin in the dark, coffee-colored urine of horses with severe rhabdomyolysis. Accurate differentiation of these compounds in urine requires more sophisticated procedures.

Cells

The normal range for cells generally is considered to be up to five red cells or leukocytes per high-power field. An increased number of erythrocytes indicates hematuria, which may be caused by neoplasia, trauma, inflammation, or coagulopathy. Pyuria, an increase in urine leukocytes, indicates an inflammatory process, and when associated with bacteriuria it indicates a septic process in the urinary tract. The presence of sheets or rafts of transitional cells suggests neoplasia.

Casts

Casts are accumulations of protein and cellular material that form in the renal tubules, and when present in the urine, they indicate renal damage or tubular disease. Casts can consist of erythrocytes, leukocytes, or renal tubular cells. As cellular degeneration proceeds, the cell type is more difficult to determine, and the casts become granular and then waxy. Hyaline casts are noncellular and are formed from mucoprotein. They may be seen with glomerulonephritis, fever with passive congestion, or severe dehydration with altered renal blood flow.

Crystals

The crystalline structures in the urine of large animals are those usually associated with an alkaline urine. Calcium carbonate crystals normally are found in abundance in horse urine, particularly if the horse has been fed alfalfa hay. Triple phosphate and calcium oxalate crystals are frequently observed in relatively small numbers. The major crystals involved in urolithiasis of feedlot cattle is struvite (Mg NH₄PO₄·6H₂). In the western United States silicate stones are most common in livestock under range conditions. Carbonate and oxalate stones are common causes of urolithiasis in small flocks of backyard sheep and goats fed alfalfa hay.

Bacteria

Bacteria sometimes are seen in small numbers in voided urine samples and may represent surface contaminants. Bacterial infections of the urinary tract usually are associated with significant pyuria. When this is noted, a catheterized sample (horse) or clean midstream catch (ruminant) should be obtained for Gram stain, culture, and sensitivity testing. Results from ruminant samples must be carefully interpreted, because male ruminants routinely urinate within the prepuce, thus heavily contaminating samples. Fortunately, urinary tract infections in male ruminants and horses are rare.

Urine Creatinine Clearance Ratio

Urinary electrolyte excretion is affected by a variety of factors, including dietary intake, alterations in renal function, and specific hormones that regulate renal electrolyte excretion Determination of electrolyte concentrations from randomly collected urine samples is easily done and can be clinically useful when these concentrations are compared with serum concentrations. The presence of substantial amounts of sodium in the urine of an animal with hyponatremia suggests excessive renal sodium loss caused by altered renal function or hormonal control. However, marked variation in the rate of urine production can lead to serious problems in the interpretation of urine electrolyte concentrations. The standard physiologic methods of determining renal electrolyte clearance are complicated by the need for quantitative urine collection and are not well suited to most practical clinical situations. One means of overcoming these difficulties is expression of the renal electrolyte clearance relative to the endogenous creatinine clearance. Expression of the renal clearance as a ratio eliminates the need for quantitative urine collection. This derived value is known as the creatinine clearance ratio or the fractional excretion (FE) and is calculated by the following formula:

where X represents the electrolyte concentration and C represents the creatinine concentration.

The FE fluctuates somewhat during the day in relation to physical activity and feed and water intake. However, under standardized conditions, there is close agreement between the FE determined from single random samples of blood and urine and that based on samples quantitatively collected during a 12- to 24-hour period.¹⁶ The FE of electrolytes has a very wide range of normal values. The principal sources of this variation are differences in dietary intake and environmental or experimental conditions. The FE of electrolytes has been useful for detecting specific dietary deficiencies or imbalances. Dietary salt deficiency is associated with an extremely low FE of sodium and chloride, whereas the plasma concentration of these ions generally remains within normal limits. In a similar fashion, dietary calcium and phosphorus imbalances seldom are reflected by the serum concentrations of these ions. Calcium deficiency and phosphorus excess are relatively common dietary problems in large animals and result in a low FE for calcium and a high FE for phosphorus. There are technical problems with the determination of the urine calcium concentration in horses because of their normally alkaline urine and the resultant precipitation of calcium carbonate in the urine. Mixed urine samples from horses must be acidified (hydrochloric acid can be used) to solubilize the calcium.

Increases in the FE of sodium are noted with renal tubular damage and impaired sodium resorption. The sodium FE increase has been a useful indicator for the differential diagnosis of prerenal azotemia, which almost invariably results in a low sodium FE, from the azotemia caused by primary renal disease, in which the FE for sodium often is markedly increased.