Normal Findings

Indications

This test is used to aid in the diagnosis and surveillance of skeletal muscle diseases.

Test Explanation

Serum aldolase is very similar to the enzymes aspartate aminotransferase (AST) (see p. 119) and creatine kinase (CK) (see p. 186). Aldolase is an enzyme used in the glycolytic breakdown of glucose. As with AST and CPK, aldolase is present in most tissues of the body. This test is most useful for identifying muscular or hepatic cellular injury or destruction. The serum aldolase level is very high in patients with muscular dystrophies, dermatomyositis, and polymyositis. Levels also are increased in patients with gangrenous processes, muscular trauma, and muscular infectious diseases (e.g., trichinosis). Elevated levels are also noted in chronic hepatitis, obstructive jaundice, and cirrhosis.

Neurologic diseases causing weakness can be differentiated from muscular causes of weakness with this test. Normal values are seen in patients with such neurologic diseases as poliomyelitis, myasthenia gravis, and multiple sclerosis. Elevated aldolase levels are seen in patients with primary muscular disorders.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Test

Normal Findings

Indications

This test is used to diagnose hyperaldosteronism. To differentiate primary aldosteronism (adrenal pathology) from secondary aldosteronism (extraadrenal pathology), a plasma renin assay must be performed simultaneously.

Test Explanation

Aldosterone, a hormone produced by the adrenal cortex, is a potent mineralocorticoid. Production of aldosterone is regulated primarily by the renin-angiotensin system. This system works as follows: a decreased effective renal blood flow triggers pressure-sensitive renal glomerular elements to release renin. The renin then stimulates the liver to secrete angiotensin I, which is converted to angiotensin II in the lung and kidney. Angiotensin II is a potent stimulator of aldosterone (see Figure 2-26, p. 448).

Secondarily, aldosterone is stimulated by adrenocorticotropic hormone (ACTH), low serum sodium levels, and high serum potassium levels. Aldosterone in turn stimulates the renal tubules to absorb sodium (water follows) and to secrete potassium in the urine. In this way, aldosterone regulates serum sodium and potassium levels. Because water follows sodium transport, aldosterone also partially regulates water absorption (and plasma volume).

Increased aldosterone levels are associated with primary aldosteronism in which a tumor (usually an adenoma) of the adrenal cortex (Conn syndrome) or bilateral adrenal nodular hyperplasia causes increased production of aldosterone. The typical pattern for primary aldosteronism is an increased aldosterone level and a decreased renin level. The renin level is low because the increased aldosterone level “turns off” the renin-angiotensin system. Patients with primary aldosteronism characteristically have hypertension, weakness, polyuria, and hypokalemia.

Increased aldosterone levels also occur with secondary aldosteronism caused by nonadrenal conditions. These include the following:

In secondary aldosteronism, aldosterone levels and renin levels are high.

The aldosterone assay can be performed on a 24-hour urine specimen or a plasma blood sample. The advantage of the 24-hour urine sample is that short-term fluctuations are eliminated. Plasma values are more convenient to sample, but they are affected by short-term fluctuations. Factors that can rapidly cause fluctuation in aldosterone levels include the following:

A 24-hour urine collection is therefore much more reliable because the effect of these interfering factors is dampened.

Primary aldosteronism can be diagnosed by demonstrating little or no increase in renin levels after aldosterone stimulation (using salt restriction as the stimulant). This is because aldosterone is already maximally secreted by the pathologic adrenal gland. Also, patients with primary aldosteronism fail to suppress aldosterone after saline infusion (1.5 to 2 L of normal saline solution infused between 8 AM and 10 AM). Aldosterone can be measured in blood obtained from adrenal venous sampling. In this situation, high levels from the right and left adrenal veins are diagnostic of bilateral adrenal hyperplasia. Unilateral high aldosterone levels are found in patients with aldosterone-producing tumors of the adrenal gland or renal artery stenosis. Renin levels are usually obtained at the same time. High unilateral renin levels with unilateral high aldosterone levels indicate renal artery stenosis. Aldosterone-producing tumors of the adrenal gland are characterized by unilateral high adrenal vein aldosterone and low renin levels.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

ALP is used to detect and monitor diseases of the liver or bone.

Test Explanation

Although ALP is found in many tissues, the highest concentrations are found in the liver, biliary tract epithelium, and bone. The intestinal mucosa and placenta also contain ALP. This phosphatase enzyme is called alkaline because its function is increased in an alkaline (pH of 9 to 10) environment. This enzyme test is important for detecting liver and bone disorders. Within the liver, ALP is present in Kupffer cells. These cells line the biliary collecting system. This enzyme is excreted into the bile. Enzyme levels of ALP are greatly increased in both extrahepatic and intrahepatic obstructive biliary disease and cirrhosis. Other liver abnormalities, such as hepatic tumors, hepatotoxic drugs, and hepatitis, cause smaller elevations in ALP levels. Reports have indicated that the most sensitive test to indicate tumor metastasis to the liver is the ALP.

Bone is the most frequent extrahepatic source of ALP; new bone growth is associated with elevated ALP levels. Pathologic new bone growth occurs with osteoblastic metastatic (e.g., breast, prostate) tumors. Paget disease, healing fractures, rheumatoid arthritis, hyperparathyroidism, and normal-growing bones are sources of elevated ALP levels as well.

Isoenzymes of ALP are also used to distinguish between liver and bone diseases. These isoenzymes are most easily differentiated by the heat stability test and electrophoresis. The isoenzyme of liver origin (ALP1) is heat stable; the isoenzyme of bone origin (ALP2) is inactivated by heat. The detection of isoenzymes can help differentiate the source of the pathologic condition associated with the elevated total ALP. ALP1 would be expected to be high when liver disease is the source of the elevated total ALP. ALP2 would be expected to be high when bone disease is the source of the elevated total ALP. Another way to separate the source of elevated ALP is to simultaneously test for 5′-nucleotidase. This later enzyme is made predominantly in the liver. If total ALP and 5′-nucleotidase are concomitantly elevated, the disease is in the liver. If 5′-nucleotidase is normal, the bone is the most probable source.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

Allergy blood testing is an alternative to allergy skin testing in diagnosing allergy as a cause of a particular symptom complex. It is also useful in identifying the specific allergen affecting a patient. It is particularly helpful when allergy skin testing is contraindicated.

Test Explanation

Measurement of serum IgE is an effective method to diagnose allergy and specifically identify the allergen (the substance to which the person is allergic). Serum IgE levels increase when allergic individuals are exposed to the allergen. Various classes of allergens can initiate the allergic response. They include animal dandruff, foods, pollens, dusts, molds, insect venoms, drugs, and agents in the occupational environment.

Although skin testing (see p. 1079) can also identify a specific allergen, measurement of serum levels of IgE is helpful when a skin test result is questionable, when the allergen is not available in a form for dermal injection, or when the allergen may incite an anaphylactic reaction if injected. IgE is particularly helpful in cases in which skin testing is difficult (e.g., in infants or in patients with dermatographism or widespread dermatitis), and it is not always necessary to remove the patient from antihistamines. The decision concerning which method to use to diagnose an allergy and to identify the allergen depends on the elapsed time between exposure to an allergen and testing, class of allergen, age of patient, the possibility of anaphylaxis, and the affected target organ (such as skin, lungs, or intestine). In general, allergy skin testing is the preferred method in comparison with various in vitro tests for assessing the presence of specific IgE antibodies because it is more sensitive and specific, simpler to use, and less expensive.

IgE levels, like provocative skin testing, are used not only to diagnose allergy, but also to identify the allergen so that an immunotherapeutic regimen can be developed. Increased levels of total IgE can be diagnostic of allergic disease in general. Specific IgE blood allergy testing, however, is an in vitro test for specific IgE directed to a specific allergen. Since the development of liquid allergen preparations, the use of in vitro blood allergy testing has increased considerably. It is more accurate and safer than skin testing.

Once the allergen has been identified, for most patients, the treatment would include avoidance of the allergen and use of bronchodilators, antihistamines, and possibly steroids. If aggressive antiallergy treatment is provided before testing, IgE levels may not rise despite the existence of an allergy.

Allergy to latex-containing products is an increasingly common allergy for which certain industrial and most medical personnel are at risk. It is an allergy that may develop in otherwise nonallergic patients because of overexposure. Furthermore, patients with latex exposure are at risk for allergic reaction if they undergo operative procedures or any procedure for which the health care personnel wear latex gloves. In these patients a latex-specific IgE can be easily identified with the use of an enzyme-labeled immunometric assay. This test is 94% accurate.

There are many methods of measuring IgE. One of the most commonly used methods is the radioallergosorbent test (RAST). In this method, the serum of a patient suspected of having a specific allergy is mixed with a specific allergen. The antibody-allergen complex is then incubated with one or more radiolabeled monoclonal anti-IgE antibodies. The total amount of IgE can be measured. Enzyme-conjugated, radioimmunometric, colorimetric, fluorometric, or chemiluminometric methods are used for allergen-specific IgE quantification. Accuracy varies between 45% and 95% depending on the allergen.

Allergy testing of IgG antibodies can also be performed and may provide a more accurate correlation between allergen and allergic symptoms. Like IgE antibody testing, IgG antibody testing is often performed in “panels.” For example, there are meat panels that might include IgE or IgG testing for chicken, duck, goose, and turkey. Testing a fruit panel might include IgE or IgG antibody testing for apples, bananas, peaches, and pears. Testing in panels diminishes the cost of testing. Specific allergen antibody testing can follow panel testing. Testing methods vary by laboratory. They may include ELISA/EIA or Quantitative Immunofluorescence Enzyme Assay.

Contraindications

Interfering Factors

Procedure

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

Serum alpha₁-antitrypsin (AAT) determinations are obtained in patients with a family history of emphysema, because there is a familial tendency for a deficiency of this anti-enzyme. Deficient or absent serum levels of this enzyme can cause the early onset of disabling emphysema. A similar deficiency in AAT is seen in children with cirrhosis and other liver diseases.

AAT is also an acute-phase reactant protein that is elevated in the presence of inflammation, infection, or malignancy. It is not specific regarding the source of the inflammatory process.

Test Explanation

AAT inactivates endoproteases (protein catabolic enzymes that are released in the body by degenerating and dying cells), such as trypsin and neutrophil elastase, that can break down elastic fibers and collagen, especially in the lung. Deficiencies of AAT can be genetic or acquired. Acquired deficiencies in AAT can occur in patients with protein-deficiency syndromes (e.g., malnutrition, liver disease, nephrotic syndrome, neonatal respiratory distress syndrome). People with AAT deficiency develop severe panacinar (although usually more severe in the lower third of the lungs) emphysema in the third or fourth decade of life. Their major clinical symptoms usually include progressive dyspnea with minimal coughing. Chronic bronchitis is prominent in those patients with deficient AAT levels who smoke. Bronchiectasis can also occur in these patients.

Inherited AAT deficiency is associated with symptoms earlier in life than acquired AAT disease. Inherited AAT is also commonly associated with liver and biliary disease. AAT Genetic Phenotyping (AAT phenotyping) has shown that most persons have two AAT “M” genes (designated as MM) and AAT levels over 250 mg/dL. “Z” and “S” gene mutations are typically associated with alterations in serum levels of AAT. Individuals who are ZZ or SS homozygous have serum levels below 50 mg/dL and often near zero.

Individuals who are MZ or MS heterozygous have diminished or low-normal serum levels of AAT. Approximately 5% to 14% of the adult population have this heterozygous state, which is considered to be a risk factor for emphysema. Homozygous individuals have severe pulmonary and liver disease very early in life. AAT Phenotyping is particularly helpful when blood AAT levels are suggestive but not definitive.

AAT is qualitatively measured using immunochemical methods. Quantification is possible but rarely useful with phenotyping. Routine serum protein electrophoresis (p. 424) is a good screening test for AAT deficiency, because AAT accounts for about 90% of the protein in the alpha₁-globulin region on electrophoresis.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

This test is used as a screening marker indicating increased risk for birth defects, such as fetal body wall defects, neural tube defects, and chromosomal abnormalities. It can also be used as a tumor marker to identify cancers.

Test Explanation

AFP is an oncofetal protein normally produced by the fetal liver and yolk sac. It is the dominant fetal serum protein in the first trimester of life and diminishes to very low levels by the age of 1 year. Normally it is found in very low levels in the adult.

AFP is an effective screening serum marker for fetal body wall defects. The most notable of these is neural tube defects, which can vary from a small myelomeningocele to anencephaly. If a fetus has an open body wall defect, fetal serum AFP leaks out into the amniotic fluid and is picked up by the maternal serum. Normally AFP from fetal sources can be detected in the amniotic fluid or the mother's blood after 10 weeks' gestation. Peak levels occur between 16 and 18 weeks. Maternal serum reflects that change in amniotic AFP levels. When elevated maternal serum AFP levels are identified, further evaluation with repeat serum AFP levels, amniotic fluid AFP levels, and ultrasound is warranted. Other examples of fetal body wall defects would include omphalocele and gastroschisis.

Elevated serum AFP levels in pregnancy may also indicate multiple pregnancy, fetal distress, fetal congenital abnormalities, or intrauterine death. Low AFP levels after correction for age of gestation, maternal weight, race, and presence of diabetes are found in mothers carrying a fetus with trisomy 21 (Down syndrome). There are other indicators of trisomy that are often performed simultaneously. See Maternal Screen Testing (p. 354) and Fetal Nuchal Translucency (p. 888).

AFP is also used as a tumor marker. Increased serum levels of AFP are found in as many as 90% of patients with hepatomas. The higher the AFP level, the greater the tumor burden. A decrease in AFP is seen if the patient is responding to antineoplastic therapy. AFP is not specific for hepatomas, although extremely high levels (above 500 ng/mL) are diagnostic for hepatoma. Other neoplastic conditions, such as nonseminomatous germ cell tumors and teratomas of the testes, yolk sac and germ cell tumors of the ovaries, and to a lesser extent Hodgkin disease, lymphoma, and renal cell carcinoma, are also associated with elevated AFP levels. Testing methods for AFP quantification include radioimmunoassay or enzyme-linked immunosorbent assay (ELISA) with a commercially available kit. Noncancerous causes of elevated AFP levels occur in patients with cirrhosis or chronic active hepatitis.

Interfering Factors

Procedure and Patient Care

If an AFP test is to be performed on amniotic fluid, follow “Procedure and Patient Care” for amniocentesis, p. 632.

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

This test is used to evaluate aluminum levels in patients with renal failure. Elevated concentrations of aluminum in a patient with an aluminum-based joint implant suggest significant prosthesis wear.

Test Explanation

Under normal physiologic conditions, the usual daily dietary intake of aluminum (5-10 mg) is completely excreted by the kidneys. Patients in renal failure (RF) lose the ability to clear aluminum and are at risk for aluminum toxicity. Aluminum-laden dialysis water and aluminum-based phosphate binder gels designed to decrease phosphate accumulation increase the incidence of aluminum toxicity in RF patients. Furthermore, the dialysis process is not highly effective at eliminating aluminum. If a significant load exceeds the body's excretory capacity, the excess is deposited in various tissues, including bone, brain, liver, heart, spleen, and muscle. This accumulation causes morbidity and mortality through various mechanisms. Brain deposition has been implicated as a cause of dialysis dementia. In bone, aluminum replaces calcium and disrupts normal osteoid formation.

Aluminum is absorbed from the GI tract in the form of oral phosphate-binding agents (aluminum hydroxide), parenterally via immunizations, via dialysate on patients on dialysis, via total parenteral nutrition (TPN) contamination, via the urinary mucosa through bladder irrigation, and transdermally in antiperspirants. Lactate, citrate, and ascorbate all facilitate GI absorption.

Serum aluminum concentrations are likely to be increased above the reference range in patients with metallic joint prosthesis. Serum concentrations >10 ng/mL in a patient with an aluminum-based implant suggest significant prosthesis wear. Chromium and other metals can be determined using similar laboratory techniques.

Interfering Factors

Procedure and Patient Care

Abnormal Findings

Normal Findings

Normal values vary for different amino acids

Indications

Measurement of certain amino acids is performed to identify diseases associated with specific essential amino acid deficiencies.

Test Explanation

Amino acids are “building blocks” of proteins, hormones, nucleic acids, and pigments. They can act as neurotransmitters, enzymes, and coenzymes. There are eight essential amino acids that must be provided to the body by the diet. The body can make the others. The essential amino acids must be transported across the gut and renal tubular lining cells. The metabolism of the essential amino acids is critical to the production of other amino acids, proteins, carbohydrates, and lipids. Amino acid levels can thereby be affected by defects in renal tubule or gastrointestinal (GI) transport of amino acids.

When there is a defect in the metabolism or transport of any one of these amino acids, excesses of their precursors or deficiencies of their “end product” amino acid are evident in the blood and/or urine. There are more than 90 diseases described that are associated with abnormal amino acid function.

Clinical manifestations of these diseases may be precluded if diagnosis is early, and appropriate dietary replacement of missing amino acids is provided. Usually urine testing (see phenylketonuria [PKU] testing, p. 374) for specific amino acids is used to screen for some of these errors in amino acid metabolism and transport. Blood testing is very accurate. Federal law now requires hospitals to test all newborns for inborn errors in metabolism, including amino acids. Testing is required for errors in amino acid metabolism such as phenylketonuria (PKU), maple syrup urine disease (MSUD), and homocystinuria. Testing for more rare disorders may include testing for tyrosinemia and argininosuccinic aciduria.

A few drops of blood are obtained from the heel of a newborn baby to fill a few circles on filter paper (Guthrie card) labeled with names of infant, parent, hospital, and primary physician. The sample is usually obtained on the second or third day of life, after protein-containing feedings (i.e., breast milk or formula) have started,

Once a presumptive diagnosis is made, amino acid levels can be determined by chromatographic methods on blood or amniotic fluid. The genetic defects for many of these diseases are becoming more defined, allowing for even earlier diagnosis to be made in utero. Common examples of amino acid diseases include PKU, cystinosis, and cystic fibrosis.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Test

Normal Findings

Indications

Ammonia is used to support the diagnosis of severe liver diseases (fulminant hepatitis or cirrhosis), and for surveillance of these diseases. Ammonia levels are also used in the diagnosis and follow-up of hepatic encephalopathy.

Test Explanation

Ammonia is a by-product of protein catabolism. Most of it is made by bacteria acting on proteins present in the gut. By way of the portal vein, it goes to the liver, where it is normally converted into urea and then secreted by the kidneys. Ammonia cannot be catabolized in the presence of severe hepatocellular dysfunction. Furthermore, when portal blood flow to the liver is altered (e.g., in portal hypertension), ammonia cannot reach the liver to be catabolized. Ammonia blood levels rise. Plasma ammonia levels do not correlate well with the degree of hepatic encephalopathy. Inherited deficiencies of urea cycle enzymes, inherited metabolic disorders of organic acids, and the dibasic amino acids lysine and ornithine are a major cause of high ammonia levels in infants and adults. Finally, impaired renal function diminishes excretion of ammonia, and the blood levels rise. High levels of ammonia result in encephalopathy and coma. Arterial ammonia levels are more reliable than venous levels but more difficult to obtain and are therefore not routinely used.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Critical Values

More than three times the upper limit of normal (depending on the method)

Indications

This test is used to detect and monitor the clinical course of pancreatitis. It is frequently ordered when a patient presents with acute abdominal pain.

Test Explanation

The serum amylase test, which is easy and rapidly performed, is most specific for pancreatitis. Amylase is normally secreted from pancreatic acinar cells into the pancreatic duct and then into the duodenum. Once in the intestine it aids in the catabolism of carbohydrates to their component simple sugars. Damage to pancreatic acinar cells (as in pancreatitis) or obstruction of the pancreatic duct flow (as in pancreatic carcinoma or common bile duct gallstones) causes an outpouring of this enzyme into the intrapancreatic lymph system and the free peritoneum. Blood vessels draining the free peritoneum and absorbing the lymph pick up the excess amylase. An abnormal rise in the serum level of amylase occurs within 12 hours of the onset of disease. Because amylase is rapidly cleared (2 hours) by the kidney, serum levels return to normal 48 to 72 hours after the initial insult. Persistent pancreatitis, duct obstruction, or pancreatic duct leak will cause persistent elevated amylase levels.

Although serum amylase is a sensitive test for pancreatic disorders, it is not specific. Other nonpancreatic diseases can cause elevated amylase levels in the serum. For example, during bowel perforation, intraluminal amylase leaks into the free peritoneum and is picked up by the peritoneal blood vessels. This results in an elevated serum amylase level. A penetrating peptic ulcer into the pancreas will also cause elevated amylase levels. Duodenal obstruction can be associated with less significant elevations in amylase. Because salivary glands contain amylase, elevations can be expected in patients with parotiditis (mumps). Amylase isoenzyme testing can differentiate pancreatic from salivary hyperamylasemia. Amylase is also found in low levels in the ovaries and skeletal muscles. Ectopic pregnancy and severe diabetic ketoacidosis are also associated with hyperamylasemia.

Patients with chronic pancreatic disorders that have resulted in pancreatic cell destruction or patients with massive hemorrhagic pancreatic necrosis often do not have high amylase levels, because there may be so few pancreatic cells left to make amylase.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

This test is performed to identify renovascular hypertension.

Test Explanation

Renin (p. 447) is an enzyme that is released by the juxtaglomerular apparatus of the kidney. Its release is stimulated by hypokalemia, hyponatremia, decreased renal blood perfusion, or hypovolemia. Renin stimulates the release of angiotensinogen. Angiotensin-converting enzyme (ACE) (p. 64) metabolizes angiotensinogen to angiotensin I and subsequently to angiotensin II and III. Angiotensin then stimulates the release of catecholamines, antidiuretic hormone, ACTH, oxytocin, and aldosterone. Angiotensin is also a vasoconstrictor. Angiotensin is used to identify renovascular sources of hypertension. It can be measured as angiotensin I or angiotensin II. The test is performed by direct radioimmunoassay.

Interfering Factors

See Plasma Renin Assay, p. 447.

Procedure and Patient Care

Test Results and Clinical Significance

Related Test

Normal Findings

Indications

ACE is used to detect and monitor the clinical course of sarcoidosis (a granulomatous disease that affects many organs, especially the lungs). Furthermore, it is used to differentiate between sarcoidosis and other granulomatous diseases. It is also used to differentiate active and dormant sarcoid disease.

Test Explanation

ACE is found in pulmonary epithelial cells and converts angiotensin I to angiotensin II (a potent vasoconstrictor). Angiotensin II is a significant stimulator of aldosterone. ACE is vital in the renin/aldosterone mechanism and therefore important in controlling blood pressure. Despite this, ACE is not very helpful in the evaluation of hypertension. Its value is in the detection of sarcoidosis.

Elevated ACE levels are found in a high percentage of patients with sarcoidosis. This test is primarily used in patients with sarcoidosis to evaluate the severity of disease and the response to therapy. Levels are especially high with active pulmonary sarcoidosis and can be normal with inactive (dormant) sarcoidosis. Elevated ACE levels also occur in conditions other than sarcoidosis, including Gaucher disease (a rare familial lysosomal disorder of fat metabolism), leprosy, alcoholic cirrhosis, active histoplasmosis, tuberculosis, Hodgkin disease, myeloma, scleroderma, pulmonary embolism, and idiopathic pulmonary fibrosis. ACE is elevated in the CSF of patients with neurosarcoidosis. An ACE assay can be performed using spectrophotometry or radioimmunoassay.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Normal Findings

Indications

Calculation of the anion gap (AG) assists in the evaluation of patients with acid-base disorders. It is used to attempt to identify the potential cause of the disorder and can also be used to monitor therapy for acid-base abnormalities.

Test Explanation

The anion gap (AG) is the difference between the cations and the anions in the extra-cellular space that are routinely calculated in the laboratory (i.e., AG = [sodium + potassium] − [chloride + bicarbonate]). In some laboratories, the potassium is not measured because the level of potassium in acid-base abnormalities varies. The normal value of the anion gap is adjusted downward if potassium is eliminated from the equation. The anion gap, although not real physiologically, is created by the small amounts of anions in the blood (such as lactate, phosphates, sulfates, organic anions, and proteins) that are not measured. Further, it is important to realize that the that is measured is actually the venous CO₂, not the arterial .

This calculation is most often helpful in identifying the cause of metabolic acidosis. As acids such as lactic acid or ketoacids accumulate in the bloodstream, bicarbonate neutralizes them to maintain a normal pH within the blood. Mathematically, when bicarbonate decreases, the AG increases. In general, most metabolic acidotic states (excluding some types of renal tubular acidosis) are associated with an increased anion gap. The higher the gap above normal, the more likely this will be the case. Proteins can have a significant effect on AG. As albumin (usually negatively charged) increases, AG will increase. In the face of normal albumin, a high AG is usually a result of an increase in non–chloride-containing acids or organic acids (such as lactic acid or ketoacids).

A decreased AG is very rare but can occur when there is an increase in unmeasured (calcium or magnesium) cations. A reduction in anionic proteins (nephrotic syndrome) will also decrease AG. For example, a 1 g/dL drop in serum protein is associated with a 2.5 mEq/L drop in AG. Because the anion proteins are lost, the increases to maintain electrical neutrality. Increase in cationic proteins (some immunoglobulins) will also decrease AG. Except for hypoproteinemia, conditions that cause a reduced or negative anion gap are relatively rare compared to those associated with an elevated anion gap.

AG measurement is also helpful in identifying the presence of a mixed acid-base situation. The arterial blood gases do not always tell the whole story, especially if there is a mixed metabolic acidosis and a concomitantly occurring alkalosis. An increased AG, despite a normal pH will indicate an acidotic component to the metabolic picture. When AG measurement is combined with the ABGs and electrolytes, complex metabolic clinical pictures can be more clearly elucidated. The AG calculation is indicated whenever an acid-base problem exists.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

This test is positive in some patients with systemic lupus erythematosus (SLE). The presence of this antibody places the patient at higher risk for “antiphospholipid syndrome” (i.e., venous or arterial thrombosis, thrombocytopenia, recurrent spontaneous abortions). This test is performed on patients with SLE to determine if the patient is at risk for developing the above-noted complications.

Test Explanation

Anticardiolipin antibodies (immunoglobulins G and M to cardiolipin) are antiphospholipid autoantibodies that attach to phospholipids of cell membranes and can interfere with the coagulation system. Antiphospholipid autoantibodies include anticardiolipin antibodies and the “lupus anticoagulant antibody.” Phospholipid antibodies occur in patients with a variety of clinical signs and symptoms, notably thrombosis (arterial or venous), pregnancy morbidity (unexplained fetal death, premature birth, severe preeclampsia, or placental insufficiency), unexplained cutaneous circulation disturbances (livido reticularis or pyoderma gangrenosum), thrombocytopenia or hemolytic anemia, and nonbacterial thrombotic endocarditis. Phospholipid antibodies and lupus anticoagulants are found with increased frequency in patients with systemic rheumatic diseases, especially lupus erythematosus. The term “antiphospholipid syndrome” (APS) or “Hughes syndrome” is used to describe the triad of thrombosis, recurrent fetal loss, and thrombocytopenia accompanied by phospholipid antibodies or a lupus anticoagulant. Both antibodies may be found in other autoimmune diseases, drug-induced lupus, and infection. These antibodies may be considered normal in the elderly person.

Antiphospholipid IgG and IgM antibodies are directed against a mixture of phosphatidylserine, phosphatidic acid, and beta-2 glycoprotein I antigens and are more specific than cardiolipin IgG and IgM antibodies in the diagnosis of antiphospholipid syndrome (APS). Testing is commonly performed by Semi-Quantitative Enzyme-Linked Immunosorbent Assay.

Interfering Factors

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

This test is used to support the diagnosis of CREST syndrome.

Test Explanation

A centromere is the region of the chromosome referred to as the primary constriction that divides the chromosome into arms. During cell division the centromere exists in the pole of the mitotic spindle.

Anticentromere antibodies are a form of antinuclear antibodies. They are found in a very high percentage of patients with CREST syndrome, a variant of scleroderma. CREST is characterized by calcinosis, Raynaud phenomenon, esophageal dysfunction, sclerodactyly (a hand deformity), and telangiectasia (permanent dilation of superficial capillaries and venules). Anticentromere antibodies, on the contrary, are present in only a small minority of patients with scleroderma, a disease that is difficult to differentiate from CREST. No correlation exists between antibody titer and severity of CREST disease.

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

Indications

This test is used to diagnose systemic lupus erythematosus (SLE).

Test Explanation

There are several chromatin antinuclear antibodies associated with autoimmune diseases. Nucleosome (NCS) represents the main autoantigen-immunogen in systemic lupus erythematosus (SLE), and these antinucleosome antibodies are an important marker of the disease activity. Antinucleosome (antichromatin) antibodies play a key role in the pathogenesis of SLE. Nearly all patients with SLE have antinucleosome antibodies. Anti-NCS is one of the many antinuclear antibodies (see p. 88) that indicate autoimmune diseases. Anti-NCS has a sensitivity of 100% and specificity of 97% for SLE diagnosis. Anti-NCS antibodies show the highest correlation with disease activity. Anti-NCS antibodies also show strong association with renal damage (glomerulonephritis and proteinuria) associated with SLE. Anti-NCS autoantibodies are more prevalent than anti-DNA in patients with SLE.

Histone antibodies are present in 20% to 55% of idiopathic SLE cases and 80% to 95% of drug-induced lupus erythematosus cases. They occur in less than 20% of other types of connective tissue diseases. This antibody is particularly helpful in identifying patients with drug-induced lupus erythematosus caused by drugs such as procainamide, quinidine, penicillamine, hydralazine, methyldopa, isoniazid, and acebutolol. There are several subtypes of AHA. In drug-induced lupus erythematosus, a specific AHA (anti-[(H2A-H2B)-DNA] IgG) is produced. In most of the other associated diseases (rheumatoid arthritis, juvenile rheumatoid arthritis, primary biliary cirrhosis, autoimmune hepatitis, and dermatomyositis/polymyositis), the AHAs are of other varying specificities. A variety of immune testing methods are used to identify these antinuclear antibodies including ELISA and indirect immunofluorescence methods.

Procedure and Patient Care

Test Results and Clinical Significance

Related Tests

Normal Findings

<20 units/mL

Indications

Anti-CCP is useful in the diagnosis of patients with unexplained joint inflammation, especially when the traditional blood test, rheumatoid factor (RF) (see p. 454), is negative or below 50 units/mL.

Test Explanation

Anti-CCP is known more formally as anticyclic-citrullinated peptide antibody and is formed by the intermediary conversion of the amino acid ornithine to arginine. Anti-CCP appears early in the course of rheumatoid arthritis (RA) and is present in the blood of most patients with the disease. When the citrulline antibody is detected in a patient's blood, there is a high likelihood that the patient has RA. Among patients with early RA, 30% to 40% may not have elevation of RF, making the diagnosis difficult in the initial stage. If the anti-CCP is elevated, the diagnosis of RA can be made even if RF is negative. This is particularly important because aggressive treatment in the early stages of RA prevents progression of joint damage. Anti-CCP may rise years before any clinical onset of arthritis or significant elevation of RF. At a cutoff of 5 units/mL, the sensitivity and specificity of anti-CCP for RA are 67.5% and 99.3%, respectively. RF has a sensitivity of 66.3% and a lower specificity (82.1%) than anti-CCP. When the two antibodies are used together, the specificity for diagnosing RA is 99.1%. Other autoimmune inflammatory diseases are rarely associated with elevated anti-CCP levels. It may also be useful in differentiating other entities that can resemble RA, and at times, cause RF positive test results (i.e., Polymyalgia Rheumatica and Parvoviral Arthropathy).

Anti-CCP is thought to be directly involved in the pathogenesis of RA. Citrullinated proteins are found in inflamed synovial tissue of patients with RA and may elicit a humoral mechanism for the joint inflammation that highlights RA. The presence of anti-CCP in RA indicates a more aggressive and destructive form of the disease. I–t is also a marker for disease progression. Some feel that anti–CCP-positive RA and anti–CCP-negative RA are clinically different disease entities—with the former having a far worse outcome. Anti–CCP-positive RA patients have more swollen joints and show more radiologic destruction than anti–CCP-negative patients with RA.

Anti-CCP ELISA testing is well correlated among the various commercially available assays with the same antigen specificity, but the numerical normal values for each assay differ widely. These methods include semi-quantitative enzyme-linked immunosorbent assay.

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ADH levels are tested in patients suspected of having diabetes insipidus or the syndrome of inappropriate ADH (SIADH). This test is often performed in patients who complain of polyuria or polydipsia and are found to have marked variations in blood and urine osmolarity or sodium levels.

Test Explanation

ADH, also known as vasopressin, is formed by the hypothalamus and stored in the posterior pituitary gland. It controls the amount of water reabsorbed by the kidney. ADH release is stimulated by an increase in serum osmolality or a decrease in intravascular blood volume. Physical stress, surgery, and even high levels of anxiety may also stimulate ADH release. On release of ADH more water is reabsorbed from the glomerular filtrate at the level of the distal convoluted renal tubule and collecting ducts. This increases the amount of free water within the bloodstream and causes highly concentrated urine.

With low ADH levels, water is excreted, thereby producing hemoconcentration and a more dilute urine. Diabetes insipidus (DI) occurs when ADH secretion is inadequate or when the kidney is unresponsive to ADH stimulation. Inadequate ADH secretion is usually associated with central neurologic abnormalities (neurogenic DI), such as trauma, tumor, or inflammation of the brain (hypothalamus). Surgical ablation of the pituitary gland will also result in the neurogenic form of DI; such patients excrete large volumes of free water in the dilute urine. Their blood is hemoconcentrated, producing a strong thirst.

Primary renal diseases may make the renal collecting system less sensitive to ADH stimulation (nephrogenic DI). Again, in this instance, dilute urine may be produced by excretion of high volumes of free water. To differentiate ADH deficiency (neurogenic DI) from renal resistance to ADH (nephrogenic DI), a water deprivation (ADH stimulation p. 979) test is performed. Water intake is restricted during this test. Urine osmolality is measured. Vasopressin is administered. In neurogenic DI, there is no rise in urine osmolality after water restriction, but there is a rise after vasopressin is given. In nephrogenic DI, there is no rise in urine osmolality after water deprivation or vasopressin administration. The diagnosis indicated by this test can be corroborated by a serum ADH level. ADH levels are low in neurogenic DI. ADH levels are high in nephrogenic DI.

High serum ADH levels are also associated with SIADH. In response to this inappropriately high level of ADH secretion, water is reabsorbed by the kidneys greatly in excess of normal amounts. Thus the patient's blood becomes very diluted and the urine is concentrated. Blood levels of important serum ions diminish, causing severe neurologic, cardiac, and metabolic alterations. SIADH can be associated with pulmonary diseases (e.g., tuberculosis, bacterial pneumonia), severe stress (e.g., surgery, trauma), CNS tumor, infection, or trauma. Ectopic secretion of ADH from neoplasm (paraneoplastic syndrome) can cause SIADH. The most common tumors associated with SIADH include carcinomas of the lung and thymus; lymphoma; leukemia; and carcinomas of the pancreas, urologic tract, and intestine. Patients with myxedema or Addison disease also can experience SIADH. Some drugs are know to cause SIADH (see Interfering Factors).

Interfering Factors

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This test is used to differentiate the syndrome of inappropriate ADH (SIADH) from other causes of hyponatremia or edematous states listed in Box 2-3.

Test Explanation

This test is used to evaluate the possibility of SIADH in patients with electrolyte abnormalities (such as hyponatremia) or edematous states. Usually this test is performed concomitantly with measurements of urine and serum osmolality. Patients with SIADH will excrete none or very little of the water load. Furthermore, their urine osmolality will never be less than 100, and the U/S ratio is greater than 100. Patients with hyponatremia, other edematous states, or chronic renal diseases will excrete up to 80% of the water load and will develop midrange osmolality results.

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The anti-DNA antibody test is useful for the diagnosis and follow-up of systemic lupus erythematosus (SLE).

Test Explanation

This antibody is found in approximately 65% to 80% of patients with active SLE and rarely in patients with other diseases. High titers are characteristic of SLE. Low to intermediate levels of this antibody may be found in patients with other rheumatic diseases and in those with chronic hepatitis, infectious mononucleosis, and biliary cirrhosis. The anti-DNA titer decreases with successful therapy and increases with exacerbation of SLE and especially with the onset of lupus glomerulonephritis. Near-negative values are seen in patients with dormant SLE. This test is semi-quantitative. Therefore small changes in antibody levels do not indicate disease activity.

The anti-DNA IgG antibody is a subtype of the antinuclear antibodies (ANAs) (p. 88). If the ANAs are negative, there is no reason to test for anti-DNA antibodies. There are two types of anti-DNA antibodies. The first and most commonly found is the antibody against double-stranded DNA (anti–ds-DNA). The second type is the antibody against single-stranded DNA (anti–ss-DNA). This is less sensitive and specific for SLE but is positive in other autoimmune diseases. These antibody-antigen complexes that occur with autoimmune disease are not only diagnostic but are major contributors to the disease process. These complexes induce the complement system, which then may cause local or systemic tissue injury.

There are several radioimmunoassay methods for measuring anti-DNA antibodies. The Farr method is the oldest and is more sensitive than the lupus erythematosus prep. It detects anti–ds-DNA and anti–ss-DNA antibodies. As a result, its specificity is not great.

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The anti-ENAs are used to assist in the diagnosis of systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) and to eliminate other rheumatoid diseases.

Test Explanation

Anti-ENAs are a type of antinuclear antibodies to certain nuclear antigens that consist of RNA and protein. The antigen is extracted from the thymus using phosphate-buffered saline solutions and therefore is sometimes referred to as saline-extracted antigen. The most common ENAs are Smith (SM) and ribonucleoprotein (RNP) types.

The anti-SM antibody is present in about 30% of patients with SLE and in about 8% of patients with MCTD diseases. However, it is not present in patients with most other rheumatoid-collagen diseases.

The anti-RNP antibody is reported in nearly 100% of patients with MCTD disease and in about 25% of patients with SLE, discoid lupus, and progressive systemic sclerosis (scleroderma). In high titers, anti-RNP is suggestive of MCTD.

The anti–Jo-1 (antihistidyl transfer synthase) antibody occurs in patients with autoimmune interstitial pulmonary fibrosis and in a minority of patients with aggressive autoimmune myositis. Two other antibodies to ENAs are anti–SS-A and anti–SS-B (see p. 98) and are used mainly in the diagnostic evaluation of Sjögren syndrome.

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This test is used to detect the presence of circulating glomerular basement membrane antibodies commonly present in autoimmune-induced nephritis (Goodpasture syndrome).

Test Explanation

Goodpasture syndrome is an autoimmune disease characterized by the presence of circulating antibodies against an antigen in the renal glomerular basement membrane and the pulmonary alveolar basement membrane. These immune complexes activate the complement system and thereby cause tissue injury. Patients with this problem usually display a triad of glomerulonephritis (hematuria), pulmonary hemorrhage (hemoptysis), and antibodies to basement membrane antigens. This is a rare form of glomerular nephritis. About 60% to 75% of patients with immune-induced glomerular nephritis have pulmonary complications.

With the use of immunohistochemistry and now with radioimmunoassay, antibodies also can be demonstrated in the glomeruli, renal tubular basement membrane, and the pulmonary capillary basement membranes. Lung or renal biopsies are required to demonstrate these antibodies in tissue. Serum assays are a faster and more reliable method for diagnosing Goodpasture syndrome, especially in patients in whom renal or lung biopsy may be difficult or contraindicated. Furthermore, serum levels can be used in monitoring response to therapy (plasmapheresis or immunosuppression).

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Negative

Indications

This test is used to differentiate multiple sclerosis from other neurologic causes of weakness. It also is used to differentiate Crohn disease from other forms of inflammatory bowel diseases.

Test Explanation

Glycans (sugars or carbohydrates) exist on the surface of cells, such as erythrocytes. Anti-glycan antibodies are immunologically directed to these sugar-containing components. Antibodies to glycans can be instigated by bacterial, fungal, and parasitic infections. The use of glycan arrays for systematic screening of patients with multiple sclerosis (MS) and inflammatory bowel disease (particularly Crohn disease) has been helpful in enabling the diagnosis and prognosis in these patients.

Utilizing enzyme linked immunosorbent assay (ELISA), these antibodies can be identified and quantified. When used with other antibodies associated with Crohn disease (such as anti-Saccharomyces cerevisiae antibody [ASCA], anti-laminaribioside carbohydrate antibody [ALCA], anti-mannobioside carbohydrate antibody [AMCA], and anti-chitobiose carbohydrate antibody [ACCA]), anti-glycan antibodies are supportive of Crohn disease over ulcerative colitis or irritable bowel disease. Furthermore, higher levels of these antibodies are associated with a more complicated course of disease.

Other anti-glycan antibodies are specific for MS patients, enabling differentiation between MS patients and patients with other neurologic diseases.

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This test is used in the evaluation of patients suspected of having autoimmune hepatitis.

Test Explanation

Autoimmune liver disease (e.g., autoimmune hepatitis and primary biliary cirrhosis) is characterized by the presence of autoantibodies including smooth muscle antibodies (SMA), antimitochondrial antibodies (AMA), and anti-liver/kidney microsomal antibodies type 1 (anti-LKM-1). Subtypes of autoimmune hepatitis (AIH) are based on autoantibody reactivity patterns. For example, the presence of smooth muscle antibodies (SMA) is consistent with the diagnosis of chronic autoimmune hepatitis. The presence of anti-liver/kidney microsomal type 1 antibodies with or without SMA is consistent with autoimmune hepatitis, type 2. The presence of anti-mitochondrial antibodies is consistent with primary biliary cirrhosis.

Anti-LKM-1 antibodies serve as a serologic marker for AIH type 2 and typically occur in the absence of SMA and antinuclear antibodies. Children often have other autoantibodies (e.g., parietal cell antibodies and thyroid microsomal antibodies). These antibodies react with a short linear sequence of the recombinant antigen cytochrome monooxygenase P450 2D6. Patients with AIH type 2 more often tend to be young, female, and have severe disease that responds well to immunosuppressive therapy.

Patients with chronic hepatitis resulting from hepatitis C can also have elevated anti-LKM-1 antibodies. The diagnosis of autoimmune liver disease cannot be made on antibody testing alone. In many instances, autoimmune liver disease panel testing, including the antibodies discussed in the preceding paragraph, is performed. Testing is performed by semi-quantitative enzyme-linked immunosorbent assay/semi-quantitative indirect fluorescent antibody.

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The AMA test is used primarily to aid in the diagnosis of primary biliary cirrhosis.

Test Explanation

AMA is an anticytoplasmic antibody directed against a lipoprotein in the mitochondrial membrane. Normally the serum does not contain AMA at a titer greater than 1:5. AMAs are found in 94% of patients with primary biliary cirrhosis or other autoimmune liver disease. This disease may be an autoimmune disease that occurs predominantly in young or middle-aged women. It has a slow progressive course marked by elevated liver enzymes, especially alkaline phosphatase and gamma-glutamyl transpeptidase, and a positive AMA test. Liver biopsy (see p. 734) is usually required to confirm the diagnosis because the AMA test can be positive in patients with chronic active hepatitis, drug-induced cholestasis, autoimmune hepatitis (e.g., scleroderma, systemic lupus erythematosus), extrahepatic obstruction, or acute infectious hepatitis. There are subgroups of AMA. The M-2 subgroup is highly specific for primary biliary cirrhosis. It is not useful in monitoring the course of the disease, however. For the AMA test, immunofluorescent assay (IFA) or enzyme-linked immunosorbent assay (ELISA) techniques can be used.

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This test is used to detect an autoimmune source of myocardial injury and disease. AMAs may be detected in rheumatic heart disease, cardiomyopathy, postthoracotomy syndrome, and after myocardial infarction (MI). This test is not only used in the detection of an autoimmune cause for these conditions but also for monitoring response to treatment.

Test Explanation

A positive AMA test is associated with several forms of heart disease. AMAs may be detected before the development of clinical symptoms of heart disease. An immunologic basis has been suspected in rheumatic heart disease for a long time. Research has now documented the presence of serum antibodies against myocardial components and deposition of immunoglobulin and complement in lesional areas. Antibodies against heart muscle are also found in 20% to 40% of patients after cardiac surgery and in a smaller number of patients after MI. These antibodies are usually associated with pericarditis that follows the myocardial injury associated with cardiac surgery or MI (Dressler syndrome). AMAs have also been detected in patients with cardiomyopathy. Their role in this latter disease is unknown.

This test may be performed by indirect immunofluorescent technique. The patient's serum is added to a rat heart muscle extract. Antigen-antibody immune complexes are identified by immunofluorescent antihuman antibodies. Positive results (increased immunofluorescence) are reported in titers.

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This blood test is used to assist in the diagnosis of Wegener granulomatosis (WG). It also is used to follow the course of the disease, monitor the response to therapy, and provide early detection of relapse.

Test Explanation

WG is a regional systemic vasculitis in which the small arteries of the kidneys, lungs, and upper respiratory tract (nasopharynx) are damaged by a granulomatous inflammation. Diagnosis can be made by biopsy of clinically affected tissue. Serologic testing plays a key role in the diagnosis of WG and other systemic vasculitis syndromes. Most patients with WG have circulating autoantibodies against neutrophil cytoplasm, which are useful in the diagnosis.

ANCAs are antibodies directed against cytoplasmic components of neutrophils. When ANCAs are detected with indirect immunofluorescence microscopy, two major patterns of staining are present: cytoplasmic ANCA (c-ANCA) and perinuclear ANCA (p-ANCA). Specific immunochemical assays demonstrate that c-ANCA consists mainly of antibodies to proteinase 3 (PR3) and p-ANCA consists of antibodies to myeloperoxidase (MPO). Using Semi-Quantitative Indirect Fluorescent Antibody/Semi-Quantitative Multi-Analyte Fluorescent Detection to characterize ANCA (rather than the pattern of immunofluorescence microscopy) is more specific and more clinically relevant; therefore, the terms proteinase 3-ANCA (PR3-ANCA) and myeloperoxidase-ANCA (MPO-ANCA) are used.

The PR3 autoantigen is highly specific (95% to 99%) for WG. When the disease is limited to the respiratory tract, the PR3 is positive in about 65% of patients. Nearly all patients with WG limited to the kidney do not have positive PR3. When WG is inactive, the percentage of positive drops to about 30%.

The MPO autoantigen is found in 50% of patients with WG centered in the kidney. It also occurs in patients with non-WG glomerulonephritis, such as microscopic polyangiitis (MPA).

P-ANCA antibodies can also differentiate various forms of inflammatory bowel disease. See also Anti-Glycan Antibodies (p. 82). P-ANCA are found in 50% to 70% of ulcerative colitis (UC) patients, but in only 20% of Crohn disease (CD) patients.

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ANAs are used to diagnose systemic lupus erythematosus (SLE) and other autoimmune diseases. These antibodies are primarily used to screen for SLE. Because almost all patients with SLE develop autoantibodies, a negative ANA test excludes the diagnosis. If the ANA test is positive, other antibody studies must be done to corroborate the diagnosis.

Test Explanation

Autoantibodies are directed to nuclear material (ANAs) or to cytoplasmic material (anticytoplasmic antibodies) (Tables 2-3 and 2-4). Many abnormal antibodies are present in patients with autoimmune (also called rheumatic or connective tissue) diseases. ANA is a group of protein antibodies that react against cellular nuclear material. ANA is quite sensitive for detecting SLE. Positive results occur in approximately 95% of patients with this disease; however, many other rheumatic diseases are also associated with ANA (Table 2-5). ANA, therefore, is not a specific test for SLE (Table 2-6). ANA can be tested as a specific antibody or as a group with nonspecific antigens.

ANA tests are performed using different assays (indirect immunofluorescence microscopy or by enzyme-linked immunosorbent assay [ELISA]) and results are reported as a titer with a particular type of immunofluorescence pattern (when positive). Low-level titers are considered negative, while increased titers are positive and indicate an elevated concentration of antinuclear antibodies.

ANA shows up on indirect immunofluorescence as fluorescent patterns in cells that are fixed to a slide and are evaluated under an ultraviolet microscope. Different patterns are associated with a variety of autoimmune disorders. When combined with a more specific subtype of ANA (see Table 2-3), the pattern can increase specificity of the ANA subtypes for the various autoimmune diseases (Figure 2-5). An example of a positive result might be: “Positive at 1:320 dilution with a homogenous pattern.” This particular test is considered positive if ANA is found in a titer with a dilution of greater than 1:32. In general, the higher the titer of a certain ANA antibody known to be associated with a certain autoimmune disease, the more likely that disease exists and the more active the disease is. As the disease becomes less active because of therapy, the ANA titers can be expected to fall.

Often the ANA test is used to screen patients with suspected SLE. If the ANA test is negative, the patient probably does not have SLE. If positive, other corroborative serologic tests are performed (see Table 2-5). About 5% of patients with SLE have a negative ANA test. In this text, the more commonly clinically used ANA subtypes are separately discussed.

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Anti–parietal cell antibody (APCA) testing is used to diagnose an autoimmune cause of pernicious anemia.

Test Explanation

Parietal cells exist in the proximal stomach and produce hydrochloric acid and intrinsic factor. Intrinsic factor is necessary for the absorption of vitamin B₁₂ (see p. 518). Anti–parietal cell antibodies are found in nearly 90% of patients with pernicious anemia. Nearly 60% of these patients also have anti–intrinsic factor antibodies. It is thought that these antibodies contribute to the destruction of the gastric mucosa in these patients. APCA is also found in patients with atrophic gastritis, gastric ulcers, and gastric cancer.

APCA is present in other autoimmune-mediated diseases such as thyroiditis, myxedema, juvenile diabetes, Addison disease, and iron-deficiency anemia. Nearly 10% to 15% of the normal population has APCA. As one ages, the incidence of having APCA increases (especially in relatives of patients with pernicious anemia).

Laboratory testing is usually done by indirect immunofluorescence using the Fluoro-Kit. With this test system, APCA can cross-react with other antibodies, especially anticellular and antithyroid antibodies. Titer levels greater than 1:240 are considered positive.

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This antibody is diagnostic for scleroderma (progressive systemic sclerosis [PSS]) and is present in 45% of patients with that disease.

Test Explanation

Scl-70 antibody is an antinuclear antibody. On the fluorescent antinuclear antibody test, using the ultraviolet microscope, a specific pattern is created for the Scl-70 antibody. The pattern is a speckled group of dots throughout the nucleus (see Figure 2-5, p. 91). The test can be performed by fluorescent testing, enzyme-linked immunosorbent assay (ELISA), and enzyme immunoassay (EIA). Progressive serial dilutions are carried out.

PSS is a multisystem disorder characterized by inflammation with subsequent fibrosis of the small blood vessels in skin and visceral organs, including the heart, lungs, kidneys, and gastrointestinal (GI) tract. A collagen-like substance is also deposited into the tissue of these organs. In general, the higher the titer of Scl-70 antibody, the more likely it is that PSS exists and the more active the disease is. As the disease becomes less active because of therapy, the Scl-70 antibody titers can be expected to fall.

The absence of this antibody does not exclude the diagnosis of scleroderma. The antibody is rather specific for PSS but is occasionally seen in other autoimmune diseases, such as systemic lupus erythematosus (SLE), mixed connective tissue disease (MCTD), Sjögren syndrome, polymyositis, and rheumatoid arthritis.

RNA polymerase III antibodies are found in 11% to 23% of patients with PSS. Patients with PSS who are positive for RNA polymerase III antibodies form a distinct serologic subgroup and usually do not have any of the other antibodies typically found in PSS patients, such as anticentromere (p. 69) or anti–Sc1-70. PSS patients with anti-RNA polymerase III have an increased risk of the diffuse cutaneous form of scleroderma, with a high likelihood of skin involvement and hypertensive renal disease. A positive result supports a possible diagnosis of PSS. This autoantibody is strongly associated with diffuse cutaneous scleroderma and an increased risk of acute renal crisis. A negative result indicates no detectable IgG antibodies to RNA polymerase III, but does not rule out the possibility of PSS (11% to 33% sensitivity).

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