The Peripheral Blood Smear

Barbara J. Bain

With the development of sophisticated automated instruments to count and characterize blood cells, Romanowsky (Wright-Giemsa or May-Grünwald-Giemsa)−stained peripheral blood smears are now performed on only a minority of blood specimens received in a hematology laboratory. Nevertheless, the blood smear remains important for a number of reasons: it can (1) verify the result of an automated instrument, (2) provide an immediate specific diagnosis, or (3) indicate a narrow range of diagnostic possibilities, permitting a focused rather than indiscriminate investigation.¹^-³ A blood film can provide a rapid diagnosis in cases in which speed is crucial, such as in acute promyelocytic leukemia, thrombotic thrombocytopenic purpura, Burkitt lymphoma, and certain infections.⁴^,⁵ Sometimes a smear provides unexpected information that is of value in patient management.

Usually, blood smears are initially interpreted by a laboratory scientist. In some countries, it is customary for clinicians to examine the blood smears of their own patients because the clinician has the final responsibility for integrating all information and making a diagnosis. However, the interpretation of a blood smear can be difficult, and a trained laboratory hematologist or hematopathologist has a major role in interpreting smears that may have been initially examined by a laboratory scientist. It is crucial that, when requesting a blood count, the clinician provides all the essential information needed to interpret the count and any associated smear. Regardless of whether the clinician examines the blood film, he or she must be able to interpret the written report issued by the laboratory. To do so, the clinician must be familiar with the terms generally used by laboratory staff and the possible significance of the abnormalities described. The most important of these terms are illustrated in Figures 157-1 through 157-20.

FIGURE 157-1 Normal peripheral blood smear. These normal red cells are described as normocytic (i.e., of normal size) and normochromic (i.e., their staining characteristics are normal). Normal erythrocytes are biconcave discs, causing them to have an area of central pallor that does not exceed one third the diameter of the cell. There are also scattered normal platelets (×1000).

FIGURE 157-2 Smear showing multiple abnormalities. There is anisocytosis, defined as an increased variation in cell size; poikilocytosis, defined as an increased variation in cell shape; and polychromasia, defined as the presence of erythrocytes with a blue tinge to their cytoplasm—indicating a young cell recently released from the bone marrow in which the polychromasia is due to the presence of RNA. Polychromatic cells are erythrocytes with a blue tinge; because polychromatic cells are larger than normal mature erythrocytes, they are known as polychromatic macrocytes (arrow). The film also shows two cells containing bluish purple Howell-Jolly bodies; these inclusions are nuclear remnants (×1000).

FIGURE 157-3 Microcytic red cells from a case of thalassemia minor. In a blood smear, a microcyte can be defined as a cell with a diameter less than that of the nucleus of a normal small lymphocyte. There are also some cells showing hypochromia, an area of central pallor that is larger than one third of the diameter of the red cell. In addition, there are two target cells, with a hemoglobinized area in the center of the area of pallor (×1000).

FIGURE 157-4 Macrocytic anemia. A macrocyte is recognized on a blood film as a cell with a diameter that is considerably greater than that of the nucleus of a small lymphocyte. In addition, this smear shows oval macrocytes (also known as macro-ovalocytes), defined as cells that are larger than normal and oval in shape (arrow). They are of considerable diagnostic importance, being characteristic of megaloblastic anemia; they can also be seen in dyserythropoiesis (×1000).

FIGURE 157-5 Hereditary spherocytosis. A spherocyte is a red cell that lacks central pallor because of its spherical shape. In hereditary spherocytosis, there are usually cells in which the central pallor is reduced rather than absent, and they are intermediate in shape between a spherocyte and a discocyte, which is an erythrocyte with the normal shape of a biconcave disc (×1000).

FIGURE 157-6 Target cells. A target cell is an erythrocyte with a hemoglobinized area in the middle of the normal area of central pallor (×1000).

FIGURE 157-7 Sickle cells. A sickle cell is a cell with a sickle or crescent shape resulting from the polymerization of hemoglobin S. These cells are seen not only in sickle cell anemia (homozygosity for hemoglobin S) but also in compound heterozygous states such as sickle cell/hemoglobin C disease and sickle cell/β-thalassemia, which also lead to sickle cell disease. This smear also shows target cells and boat-shaped cells with a lesser degree of polymerization of hemoglobin S than in a classic sickle cell (×1000).

FIGURE 157-8 Red cell fragmentation. Red cell fragments or schistocytes are defined as fragments of erythrocytes. In addition to small angular fragments, there may be microspherocytes, cells of reduced size and spherical in form (also known as spheroschistocytes), and keratocytes, cells with two horn-like projections. Keratocytes can result from removal of a Heinz body, as well as from red cell fragmentation. Some schistocytes are referred to as helmet cells because of their typical shape. Schistocytes are seen in microangiopathic hemolytic anemias and in mechanical hemolysis (×1000).

FIGURE 157-9 Hereditary elliptocytosis. An elliptocyte is an elliptical red cell. When seen in the numbers present in this smear, they are indicative of hereditary elliptocytosis; smaller numbers are seen in other conditions such as iron deficiency anemia, in which they are sometimes referred to as pencil cells (×1000).

FIGURE 157-10 Hereditary pyropoikilocytosis. This smear shows striking poikilocytosis, including elliptocytes, microspherocytes and other fragments, and teardrop cells. This congenital condition, which is related to hereditary elliptocytosis, usually results from the inheritance of two different mutated genes from the two parents and is characterized by a severe hemolytic anemia (×1000).

FIGURE 157-11 Teardrop poikilocytes. Teardrop poikilocytes, or dacrocytes, are teardrop-shaped red cells; they are characteristic of primary myelofibrosis but are also seen in megaloblastic anemia (×1000).

FIGURE 157-12 Stomatocytosis. A stomatocyte is a cell that appears to have a central mouth-shaped or slit-like stoma. Among the less common causes is hereditary stomatocytosis. Alcohol and hydroxycarbamide therapy are more common causes (×400).

FIGURE 157-13 Numerous Pappenheimer bodies. A Pappenheimer body is an iron-containing red cell inclusion (arrow). It is smaller and more angular than a Howell-Jolly body and stains navy blue rather than purple. Pappenheimer bodies are seen following splenectomy and in sideroblastic anemias (×1000).

FIGURE 157-14 Basophilic stippling. Basophilic stippling means that there are fine (as in this case) or coarse purplish blue dots dispersed through the red cell (arrow). They are a very nonspecific feature occurring in thalassemia trait, lead poisoning, pyrimidine 5′-nucleotidase deficiency, and dyserythropoiesis in general (×1000).

FIGURE 157-15 Rouleaux formation. Rouleaux are stacks of red cells, often compared to stacks of coins. They result from an increase of high-molecular-weight globulins in the plasma, either as a reactive change or as a result of secretion of a paraprotein in a plasma cell neoplasm (×1000).

FIGURE 157-16 Red cell agglutination. Red cell agglutinates are irregular aggregates of red cells, as seen in Mycoplasma pneumoniae infection. They are also seen in other infections, such as infectious mononucleosis, and in chronic cold hemagglutinin disease (×100). (Courtesy of Jean Schafer.)

FIGURE 157-17 Homozygous hemoglobin C. Three hemoglobin C crystals are present, one indicated by an arrow. Hemoglobin C crystals are usually six-sided, with a long axis having parallel edges (×1000).

FIGURE 157-18 Toxic granulation. Toxic granulation refers to heavy staining of azurophilic granules of neutrophils. When accompanied by neutrophil vacuolation, it is often indicative of infection, but it can also result from inflammation, tissue damage, and normal pregnancy (×1000).

FIGURE 157-19 Döhle body. A Döhle body (arrow) is a pale blue-gray amorphous inclusion near the cell membrane of a neutrophil. Döhle bodies can result from infection and inflammation. Similar but different inclusions (larger and more angular) are seen in the May-Hegglin anomaly (×1000).

FIGURE 157-20 Pelger-Huët anomaly. A Pelger-Huët anomaly is a cytologic abnormality of neutrophils in which there is hypolobulation of nuclei and increased chromatin clumping. Nuclei may have a shape resembling a peanut or a pince-nez, as in the examples shown. The Pelger-Huët anomaly is inherited, but similar Pelger neutrophils are seen in myelodysplastic syndromes, in which the neutrophils may also be hypogranular (×1000). They can also be seen as an acquired reversible phenomenon in response to specific drugs, including tacrolimus and mycophenolate mofetil.

Reasons for Performing a Blood Smear

A blood smear may be requested by a clinician or initiated by a laboratory scientist or a laboratory hematologist. Clinical findings that should lead a clinician to request a blood smear are summarized in Table 157-1.

TABLE 157-1

CLINICAL FEATURES SUGGESTING THE NEED FOR A BLOOD SMEAR

CLINICAL FEATURE	REASON TO PERFORM A BLOOD SMEAR
Lymphadenopathy or splenomegaly	May be indicative of infectious mononucleosis or another reactive condition, or of leukemia or lymphoma
Clinically evident anemia	Helps in the differential diagnosis
Bruising or bleeding tendency, including unexplained retinal hemorrhages	May confirm thrombocytopenia or show morphologically abnormal platelets (which may have defective function); sometimes shows acute leukemia or other condition causing bone marrow failure
Acute renal failure	Hemolytic-uremic syndrome and thrombotic thrombocytopenic purpura should be confirmed or excluded
Jaundice and hypertension in a pregnant woman	May show schistocytes, supporting a diagnosis of HELLP syndrome
Bone pain	May indicate multiple myeloma, bone marrow infiltration, or sickle cell disease
Unexplained chest or abdominal pain or acute splenic enlargement in a child	Possible sickle cell disease
Unexplained hyperbilirubinemia	Assessment of possible hemolysis

Laboratory scientists and physicians may initiate a blood smear that has not been requested by the clinician if the clinical details indicate the possibility of a significant hematologic abnormality. However, they are also likely to evaluate a blood smear in response to abnormalities revealed by an automated instrument. These abnormalities may be quantitative or qualitative. Quantitative abnormalities that require evaluation include anemia, polycythemia, macrocytosis, microcytosis, neutrophilia, lymphocytosis, eosinophilia, thrombocytopenia, and thrombocytosis.

Modern automated instruments are able to “flag” the presence of qualitative abnormalities that require a blood film to be examined for confirmation of the abnormality or for further elucidation. “Flags” are generated in response to the electrical impedance or the light scattering characteristics of individual cells. Some instruments are dependent on cytochemical reactions of cells or on the cells' ability to polarize light. Most instruments can indicate the possibility of the presence of blast cells, reactive or other atypical lymphocytes, granulocyte precursors, or nucleated red blood cells. Some instruments can enumerate nucleated red blood cells. Instruments using a cytochemical reaction for peroxidase to help identify neutrophils, eosinophils, and monocytes may flag the appearance of large, unstained (i.e., peroxidase-negative) cells; such cells may indicate a harmless inherited peroxidase deficiency, but sometimes such cells are lymphoma cells, reactive lymphocytes, or leukemic blast cells. Instruments often flag the possibility of an erroneous platelet count, such as when there is an overlap in size between platelets and red cells or when light-scattering characteristics suggest the presence of platelet aggregates (a potential cause of pseudothrombocytopenia). A reported increase in basophil count should generally also be regarded as a flag because it often represents a pseudobasophilia, resulting from the presence of leukemia or lymphoma cells. Some instruments alert the instrument operator to the possible presence of malaria parasites.

International consensus guidelines indicate which automated instrument results require blood smear review. Whether a review is needed is determined in part by whether that specimen is the first one obtained from that patient and whether there has been a significant change from a previously validated result (referred to as a delta check). Laboratory computers can be programmed to indicate when a result meets the criteria for smear review.

Automated red cell measurements can, to some extent, replace examination of the blood film.⁶ An increased red cell distribution width indicates the presence of anisocytosis. A decreased mean cell volume is usually a reliable indicator of microcytosis. A reduction in the mean cell hemoglobin concentration indicates hypochromia (for most instruments, however, this measurement is less sensitive than the human eye in the detection of hypochromia). An increased mean cell volume usually indicates macrocytosis, but examination of a smear is necessary both to confirm that the result is not artifactual and to elucidate the cause. Some instruments can measure the hemoglobin concentration in individual cells and thus flag the presence of hyperdense cells; however, a blood film is still necessary to distinguish spherocytes from irregularly contracted cells, sickle cells, and other cells that have an increased hemoglobin concentration. Most instruments produce a histogram of the size distribution of red cells, and some do the same for the distribution of hemoglobin concentration; either of these graphic representations may show dimorphic red cells (i.e., two populations of cells).

The Blood Smear in the Differential Diagnosis of Anemia

Microcytic Anemias

In microcytic anemias (Chapter 159), the automated count is of considerable importance and may permit a distinction between iron deficiency and thalassemia heterozygosity. In iron deficiency, there is initially a normocytic normochromic anemia; only when the deficiency becomes more severe is there microcytosis. Conversely, in β-thalassemia heterozygosity, there is usually a normal or near-normal hemoglobin concentration, but the red blood cell count is increased, and there is marked microcytosis (low mean cell volume) together with a parallel reduction in mean cell hemoglobin. The blood film provides supplementary information that can favor one diagnosis or the other. Iron deficiency is more likely to be associated with hypochromia and elliptocytes (“pencil cells”), whereas in β-thalassemia heterozygosity, there is microcytosis, hypochromia is less marked, and there are more likely to be target cells and basophilic stippling. Individuals with α-thalassemia involving the deletion of two α genes (–α/–α) or (– –/α α) have red cell indices similar to those of β-thalassemia heterozygosity; in this case, the blood film usually does not provide any additional diagnostically useful information, although individuals with nondeletional α-thalassemia due to hemoglobin Constant Spring have prominent basophilic stippling. When there is deletion of a single α gene, the blood count is either less abnormal or normal, and the blood smear does not provide any extra diagnostically useful information. The blood smear is, however, a useful supplement to the blood count in suggesting a diagnosis of hemoglobin H disease (Chapter 162). The count shows anemia, marked microcytosis (low mean cell volume and mean cell hemoglobin), and usually a reduction in the mean cell hemoglobin concentration. The smear usually shows marked poikilocytosis in addition to microcytosis, and there may be polychromasia, correlating with an elevated reticulocyte count. Iron deficiency anemia also needs to be distinguished from anemia of chronic disease. The blood counts may be quite similar, but in anemia of chronic disease, the smear often shows features of inflammation, such as increased rouleaux formation, background staining (as a result of increased plasma proteins), and sometimes neutrophilia. Other rare microcytic anemias that must be distinguished from iron deficiency include congenital sideroblastic anemia (Chapter 159) and lead poisoning. In congenital sideroblastic anemia, the film is dimorphic, with one population of hypochromic microcytes and another of normochromic normocytic cells. In lead poisoning, the presence of basophilic stippling and polychromasia in a patient with microcytosis can suggest the diagnosis.

Macrocytic Anemias

A blood film can be important in distinguishing true macrocytosis from factitious macrocytosis as a result of the presence of red cell agglutinates (see Fig. 157-16). Diagnostic features that can suggest the cause of macrocytosis are shown in Table 157-2.

TABLE 157-2

USEFUL FEATURES FOR DETERMINING THE CAUSE OF MACROCYTIC ANEMIA

CAUSE	SMEAR FEATURES
Megaloblastic anemia (vitamin B₁₂ or folic acid deficiency)	Oval macrocytes, teardrop poikilocytes, hypersegmented neutrophils; when severe, marked anisocytosis and poikilocytosis, which may include red cell fragments
Ethanol excess	Target cells and stomatocytes; anisocytosis and poikilocytosis less than in megaloblastic anemia
Liver disease	Target cells, stomatocytes
Myelodysplastic syndromes, including sideroblastic anemias	Other dysplastic features such as hypogranular and hypolobulated neutrophils; if erythropoiesis is sideroblastic, a population of hypochromic microcytic cells and Pappenheimer bodies
Chronic hemolytic anemia	Polychromasia; characteristic poikilocytes sometimes present (e.g., irregularly contracted cells if there is an unstable hemoglobin)

A smear is particularly important in evaluating the possibility of a megaloblastic anemia (Chapter 164) (Figs. 157-4 and 157-21). Sometimes assays of vitamin B₁₂ and folate are normal despite a deficiency, and only the blood film features suggest the true diagnosis and indicate the need for further investigation.

FIGURE 157-21 Hypersegmented neutrophils. A hypersegmented neutrophil is a neutrophil with more than five nuclear segments or lobes, as in this example from a patient with megaloblastic anemia. Neutrophil hypersegmentation is also said to be present if there are increased numbers of neutrophils with five lobes or if the median lobe count is increased (×1000).

Normocytic Normochromic Anemia

A blood smear is only occasionally useful in determining the cause of a normocytic normochromic anemia. Signs of inflammation may be present in anemia of chronic disease. Increased rouleaux formation and background staining can also indicate multiple myeloma. Polychromasia suggests the possibility of young red cells as a result of recent blood loss or hemolysis. Small numbers of acanthocytes may indicate hypothyroidism or anorexia nervosa. Dysplastic features, such as hypogranular or pseudo-Pelger neutrophils, suggest a myelodysplastic syndrome.

Hemolytic Anemias

The possibility of hemolysis is suggested by the presence of polychromasia and macrocytosis. Specific causes of hemolysis are suggested by the presence of various poikilocytes, as shown in Table 157-3.

TABLE 157-3

BLOOD SMEAR FEATURES SUGGESTING A SPECIFIC CAUSE OF INHERITED OR ACQUIRED HEMOLYTIC ANEMIA

BLOOD SMEAR FEATURES	CONDITIONS SUGGESTED
Spherocytes	Hereditary spherocytosis, autoimmune hemolytic anemia, alloimmune hemolytic anemia (e.g., hemolytic disease of the newborn, delayed hemolytic transfusion reaction), drug-induced immune hemolytic anemia, Clostridium perfringens sepsis
Elliptocytes	Hereditary elliptocytosis
Oval macrocytes plus stomatocytes	South-East Asian ovalocytosis
Irregularly contracted cells	Glucose-6-phosphate dehydrogenase deficiency, oxidant damage from chemicals or drugs in individuals with normal red cell enzymes (e.g., dapsone administration), liver failure due to Wilson disease (release of copper from liver), unstable hemoglobin, hemoglobin C homozygosity
Sickle cells and boat-shaped cells	Sickle cell disease (e.g., sickle cell anemia or compound heterozygous states such as S/C, S/D-Punjab, S/O-Arab, S/β-thalassemia)
Target cells	Hemoglobin C homozygosity, other hemoglobinopathies, hereditary xerocytosis
Stomatocytes	Hereditary stomatocytosis
Acanthocytes	Liver failure (spur cell hemolytic anemia)
Basophilic stippling	Lead poisoning, pyrimidine 5′-nucleotidase deficiency
Red cell fragments (schistocytes)	Microangiopathic hemolytic anemia (including hemolytic-uremic syndrome, thrombotic thrombocytopenic purpura, HELLP syndrome, and sometimes hemolysis associated with disseminated intravascular coagulation), mechanical hemolytic anemia (e.g., defective cardiac prosthetic valve, march hemoglobinuria)

The distinction between spherocytes and irregularly contracted cells is important; both are dense cells with absent central pallor, but the differential diagnosis is quite different. Recognition of the features of oxidant damage is important in diagnosing glucose-6-phosphate dehydrogenase (G6PD) deficiency because sometimes an assay for G6PD performed during an acute hemolytic episode is normal (Chapter 161). In addition to irregularly contracted cells, there may be ghost cells, hemi-ghost cells (“blister cells”), and even Heinz bodies protruding from the red cells and confirmed on a Heinz body preparation. The observation of these features is an indication to repeat the assay when the acute hemolytic episode is over.

Assessment of Thrombocytopenia, Thrombocytosis, and Platelet Morphology

A blood smear is essential to validate the cell count whenever an automated count shows thrombocytopenia (e.g., a count <60 × 10⁹/L) (Chapter 172). This should be done quickly before patient management is altered, such as by postponing surgery or initiating further diagnostic workup. A platelet transfusion should never be given for an unexpected thrombocytopenia without microscopic confirmation of the count. A factitiously low platelet count is often the result of in vitro platelet aggregation (Chapter 171) and is occasionally the result of platelet satellitism, possibly also with platelet phagocytosis. To detect aggregates reliably, the edges and the tail of the smear should be examined. The presence of fibrin strands also suggests the activation of coagulation and an erroneous platelet count.

If a low platelet count is confirmed, the film may give clues to the cause (Chapter 172). Giant platelets (Figs. 157-22 and 157-23) occur in a number of inherited thrombocytopenias, including Bernard-Soulier syndrome and MYH9-related disorders (the May-Hegglin anomaly and related conditions). Small platelets are less common but are a feature of Wiskott-Aldrich syndrome and familial platelet disorder with propensity to myeloid malignancy. Agranular platelets occur in the gray platelet syndrome⁷ and platelets with a reduced number of larger than normal granules are seen in Jacobsen/Paris-Trousseau syndrome. The presence of May-Hegglin inclusions (Döhle-like bodies; see Fig. 157-19) in neutrophils indicates that the cause of the thrombocytopenia is a mutation in MYH9. In acquired thrombocytopenias, increased platelet turnover is often accompanied by the presence of large platelets, whereas bone marrow failure is associated with platelets of normal size. It is important to look for red cell fragments to confirm or exclude a diagnosis of thrombotic thrombocytopenic purpura and atypical hemolytic-uremic syndrome in any patient with the apparent recent onset of thrombocytopenia; because platelet transfusions are usually contraindicated in these conditions, the smear should be examined before platelet transfusion is contemplated. The smear of any patient with the apparent recent onset of severe thrombocytopenia should be examined carefully for evidence of acute promyelocytic leukemia; the leukemic cells may be infrequent in the circulating blood. Hemorrhagic manifestations and a low platelet count can also be indicative of meningococcal septicemia; in some patients, organisms are seen in the blood smear and the diagnosis is confirmed; in other patients, only marked toxic changes in neutrophils are detected.

FIGURE 157-22 Normal-sized platelet (arrow). Platelets have central granules, although this is not apparent in this photomicrograph (×1000).

FIGURE 157-23 Giant platelet (arrow). Giant platelets are as large as or larger than normal red cells. Giant platelets can indicate increased platelet turnover or an inherited or acquired defect in thrombopoiesis (×1000).

Thrombocytosis should also be confirmed on a smear. Factitiously elevated counts may be the result of the presence of red cell fragments (in microangiopathic or mechanical hemolytic anemia, burns, or accidental in vitro heating of the blood sample), white cell fragments (in acute leukemia and, less often, in lymphoma), cryoglobulin precipitates, or microorganisms (particularly Candida species). If the count is confirmed, the blood smear may be useful to indicate a likely cause (e.g., features of hyposplenism or the presence of basophilia in a myeloproliferative disorder).

It is sometimes necessary to examine a smear to confirm that an apparently normal platelet count is valid. This should always be done in patients with acute leukemia and an elevated white cell count; the presence of white cell fragments of a similar size to platelets can suggest that the platelet count is at a safe level when it is in fact dangerously low. Counting the ratio of platelets to other particles of similar size permits the count to be corrected. Any unexpectedly normal count should be confirmed; for example, the sudden rise of the automated platelet count in a patient being treated for a hematologic neoplasm may be the result of fungi that have colonized an indwelling intravenous line and are being shed into the blood stream.

Leukocytosis and Leukopenia

The finding of leukocytosis is not necessarily an indication for a blood smear (Chapter 170). For example, this finding would be expected in a patient with infection or following surgery or trauma, in which case smear confirmation is not required. However, unexpected leukocytosis requires a smear. Artifactual elevation is unusual but can occur as a result of cryoglobulinemia, hyperlipidemia, or the presence of Candida species. Distinguishing reactive changes from leukemia on the basis of morphology is usually straightforward for myelocytosis but more difficult for lymphocytosis.⁸ In reactive leukocytosis, there is usually toxic granulation, and Döhle bodies may be present (see Figs. 157-18 and 157-19). Vacuolation is particularly characteristic of bacterial infection, and there may be some degranulation of neutrophils. The hematologist should be aware of the changes induced by granulocyte colony-stimulating factor so that they are not confused with a response to infection; this cytokine can cause toxic granulation, Döhle bodies, vacuolation, and the presence of macropolycytes (giant neutrophils) and circulating neutrophil precursors. The changes typical of leukemia are discussed later.

Leukopenia (Chapter 167) usually requires a film for confirmation and elucidation. The exception is when it is expected in a given clinical context, such as when a patient has had recent chemotherapy. Rarely, an apparent leukopenia is artifactual, owing to the aggregation of neutrophils mediated by an autoantibody or resulting from infection-induced changes in adhesion molecules of the leukocyte surface membrane.

With some automated instruments, it is necessary to confirm that neutropenia is real. If the automated count is based on peroxidase cytochemistry, the presence of an inherited deficiency will lead to an apparent neutropenia associated with an increase of large, unstained (i.e., peroxidase-negative) cells. The scatter plot is characteristic, but because the same features could be due to acute leukemia with neutropenia and circulating blast cells, a smear is needed for confirmation.

Leukemias and Lymphomas

The blood smear is critical in the diagnosis of leukemias and lymphomas. The lymphoblasts of acute lymphoblastic leukemia are usually medium-sized agranular cells with relatively scanty cytoplasm (Fig. 157-24), whereas in acute myeloid leukemia, blast cells are generally larger, with more plentiful cytoplasm that may contain granules or Auer rods (Fig. 157-25). Myeloid blast cells vary in appearance according to whether they are myeloblasts or monoblasts. Myeloblasts are usually medium sized and may have plentiful granules, scanty granules, or no visible granules; they may contain Auer rods. Monoblasts are much larger cells with plentiful cytoplasm containing few granules and, very rarely, Auer rods. Megakaryoblasts are present in some patients and are sometimes cytologically distinctive because of their tendency to form cytoplasmic blebs or develop platelet-type granules.

FIGURE 157-24 Acute lymphoblastic leukemia. Numerous agranular blast cells with a high nuclear-to-cytoplasmic ratio are present. Platelets are decreased in number (×1000).

FIGURE 157-25 Auer rod. An Auer rod (arrow) is a rod-shaped inclusion in the cytoplasm of cells of myeloid lineage formed by the crystallization of azurophilic granule constituents. Auer rods are seen only in acute myeloid leukemia and high-grade myelodysplastic syndromes. They are usually seen in blast cells but are occasionally found in maturing cells (×1000).

Chronic myelogenous leukemia has a very characteristic blood smear (Fig. 157-26) in which the most numerous cells are myelocytes and mature neutrophils. Eosinophils and basophils are also present. Dysplastic features are generally absent. In atypical Philadelphia chromosome–negative chronic myeloid leukemia, monocytosis is more frequent and dysplastic features are present. Chronic myelomonocytic leukemia is characterized by increased monocytes, some of them immature, with inconspicuous dysplastic features and infrequent granulocyte precursors.

FIGURE 157-26 Chronic myelogenous leukemia. In this view there are myeloblasts, a myelocyte, a basophil, and a segmented neutrophil (×1000).

Chronic lymphocytic leukemia also has a very characteristic blood film, with an increase of small, mature lymphocytes of rather uniform appearance. The chromatin is often irregularly clumped, creating a mosaic effect (Fig. 157-27). Smear cells are almost always increased in number but are not pathognomonic.

FIGURE 157-27 Chronic lymphocytic leukemia. There are large numbers of rather monotonous, mature small lymphocytes with chromatin clumping. Smear cells, reflecting the mechanical fragility of the cells, are characteristic but are not seen in this photomicrograph (×1000).

Lymphoma in leukemic phase often has cytologic features that aid in the diagnosis. Follicular lymphoma, Burkitt lymphoma, and splenic marginal zone lymphoma can all be distinctive.

The Incidental Detection of Clinically Significant Abnormalities

Sometimes the examination of a blood film reveals unexpected but clinically significant information. Examples are given in Table 157-4. Detection of microorganisms is particularly likely in patients with acquired immunodeficiency syndrome (AIDS), hyposplenic patients,⁹ and patients with overwhelming sepsis, but sometimes they are detected in immunologically normal patients with only trivial symptoms. It should be noted that microorganisms in blood smears sometimes represent contaminants, particularly if specimens have been obtained by skin prick or from the umbilical cord and if there has been a delay in making the film.

TABLE 157-4

INCIDENTAL BUT CLINICALLY RELEVANT BLOOD SMEAR OBSERVATIONS

OBSERVATION	POSSIBLE SIGNIFICANCE
Acanthocytes	Abetalipoproteinemia, neuroacanthocytosis (includes choreoacanthocytosis, McLeod phenotype, Huntington-like disease 2 and pantothenate-kinase associated neurodegeneration)
Howell-Jolly bodies, target cells, and acanthocytes	Hyposplenism (congenital, previous splenectomy, celiac disease, amyloidosis)
Cryoglobulin	Hepatitis C, multiple myeloma, Waldenström macroglobulinemia
Vacuolated lymphocytes	Inherited metabolic disorders
Parasites (e.g., malaria parasites, Babesia, microfilaria, trypanosomes, Leishmania)	Parasitic infection
Fungi (Candida spp, Histoplasma capsulatum, Penicillium marneffei, Cryptococcus neoformans, Malassezia furfur)	Disseminated fungal infection or, in the case of Candida, colonization of an indwelling intravenous line
Bacteria (e.g., pneumococcus, meningococcus, Capnocytophaga canimorsus, Borrelia, Ehrlichia, Anaplasma, Yersinia pestis)	Bacterial infection
Leukoerythroblastic blood film	Bone marrow infiltration (e.g., metastatic malignancy)