This procedure gives a vast amount of information. It allows visual estimation of the amount of hemoglobin in the RBCs and the overall size of the RBCs. In addition, alterations in size, shape, and structure of individual RBCs or WBCs are visible, which may have diagnostic significance in certain diseases. Pathologic early forms of the blood cells are also visible. Finally, a good estimate of the platelet count can be made in most cases from the peripheral smear alone (normal is 7-25 platelets per oil immersion field, using 10 x oculars).
Peripheral Smear Abnormalities
Hypochromia (an increase in the RBC central clear area) raises the question of chronic iron deficiency.
Macrocytes in considerable numbers suggest a differential diagnosis of megaloblastic anemia versus the other etiologies of increased MCV (see Table 2-1). Macrocytes in small numbers suggest reticulocytosis as the most likely cause. Some hematologists differentiate between types of macrocytes: oval macrocytes (folic acid or vitamin B12 deficiency; myelodysplasia; myeloid metaplasia) and round macrocytes (alcoholism; cirrhosis; hypothyroidism; reticulocytosis; aplastic anemia).
Microcytes are small RBCs caused by any of the conditions listed under decreased MCV (see Table 2-2). Hypochromic microcytes suggest a differential diagnosis of chronic iron deficiency versus thalassemia minor or anemia of chronic disease.
Causes of Spherocytosis
ABO hemolytic disease of newborn
Acute transfusion reactions (especially ABO type)
Transfused stored bank blood
Autoimmune hemolytic anemia
Thermal injury, especially in first 24 hr
Physical RBC injury (as a component of microangiopathic hemolytic anemia)
Toxins (Clostridium welchii sepsis and certain snake venoms)
Hereditary elliptocytosis (10%-20% of cases)
Occasionally in severe Heinz body hemolytic anemias
Spherocytes are a type of microcyte in which the cell is round and lacks the central clear area. Spherocytes are a feature of congenital spherocytosis and are also found to varying degrees in certain other conditions.
Polychromatophilic RBCs (discussed later) are reticulocytes, a sign of markedly increased RBC production, and therefore may be present in patients with acute bleeding, hemolytic processes, hematopoietic and nonhematopoietic malignancies, and factor deficiency anemia responding to therapy (see Reticulocytosis etiology box below). Sometimes a few polychromatophilic RBC may be present without a definite etiology.
Schistocyte (“broken cell”; also called schizocyte) is a term given to a deformed or broken RBC. In general, the cells are smaller than normal, are misshapen, and have one or more sharp points protruding from the periphery. Names have been applied to various subgroups of misshapen RBCs, such as “burr cell,” “acanthocyte,” and “helmet cell.” Unfortunately, some of these names, especially burr cell, have been applied to different cells by different investigators, so that in most instances it might be preferable to use the noncommittal term, schistocyte. Some conditions associated with schistocytes are listed in Table 2-5.
Table 2-5 Some conditions associated with schistocytes
Most Common Causes of Reticulocytosis
Hemolytic anemia, chronic or acute (antibody induced, drug-induced, associated with abnormalities in Hb or RBC structure, etc.)
After treatment of vitamin B12/folate/iron deficiency
Acanthocytes are a subgroup of schistocytes consisting of small spherical cells with several finger-like projections from the RBC surface distributed in an irregular manner. The ends of the projections tend to be slightly thickened. Acanthocytes are typically found in large numbers in hereditary abetalipoproteinemia, in moderate numbers in severe liver disease or in anorexia nervosa, and in small numbers in association with schistocytes of other types in other conditions.
Red blood cell crenation (echinocytes) are RBCs that appear normal except for uniform small triangular projections arranged in a uniform manner around the circumference of the cell, like the outer edge of a gearwheel. When most of the RBCs have this appearance, they are most commonly artifactual; but in lesser numbers they may be found in liver disease, renal disease, hyperlipidemia, and in some RBC enzymopathies.
Bite cells (degmacytes) are RBCs with a semicircular defect in one area of the outer edge. When present in significant number, bite cells are suggestive of hemolytic anemia due to an oxidizing agent (Heinz body anemia).
Sickle cells are crescent-shaped RBCs pointed at one or both ends found in some patients with homozygous sickle cell anemia. Hemoglobin SC disease may sometimes display stubby sickled cells with a short thick bar protruding from the center that represents an Hb C crystal.
Elliptocytes (ovalocytes) are oval RBCs found in varying numbers in persons with congenital elliptocytosis and occasionally in small numbers in normal persons. When seen on edge, the cells may look somewhat like short rods and, rarely, may superficially resemble an atypical sickle cell.
Target cells consist of a peripheral ring and central disk of Hb. Target cells are found in large numbers in Hb C disease and in lesser numbers with certain other abnormal hemoglobins, in thalassemia, and in chronic liver disease.
Teardrop cells look like RBCs in which one side has been gently pulled out to a sharp point while the opposite side is still rounded. These cells are most characteristically associated with myeloid metaplasia (myelofibrosis, Chapter 7) but can also be present in lesser numbers in other myeloproliferative syndromes, such as chronic myelocytic leukemia.
Stomatocytes are RBCs that have a rectangular or slit-like central pallor configuration. This may be due to hereditary stomatocytosis or may be drug induced. A few stomatocytes may be found in normal persons and in a variety of diseases.
Rouleaux are RBCs partially adhering to each other with the overall appearance of a partially spread out stack of coins. The RBC central clear area is usually absent. This appearance is similar to that normally seen in the very thick areas of a peripheral blood smear. However, with rouleaux there are a moderate number of free single RBCs intermingled with the RBC stacks, whereas there are no free RBCs in thick areas of the smear. Considerable rouleaux formation suggests the possibility of abnormal serum proteins (such as the monoclonal proteins of multiple myeloma).
Red Blood Cell Inclusions (Fig. 2-1)
Basophilic stippling describes a moderate number of small dark blue dotlike structures scattered fairly uniformly throughout the hemoglobinated area of the RBC. Stippling is derived from nuclear remnants, so that the cell represents a reticulocyte and thus may be seen in many of the same conditions as polychromatophilic RBCs. However, stippling is especially associated with lead poisoning.
Fig. 2-1 Abnormal RBC. A, normal RBC; B, spherocyte; C, target cell; D, elliptocyte; E, echinocyte; F, sickle cell; G, stomatocyte; H, acanthocyte; I, J, K, L, schistocytes; M, teardrop RBC; N, distorted RBC with Hb C crystal protruding; O, degmacyte; P, basophilic stippling; Q, pappenheimer bodies; R, howell-Jolly body.
Howell-Jolly bodies are small, round, blue-black inclusions that are considerably larger than basophilic stippling and ordinarily occur only one to an RBC. Howell-Jolly bodies may be present in any severe anemia but are more likely to be seen in severe hemolytic anemias and after splenectomy.
Pappenheimer bodies are small dark-staining granular inclusions that tend to occur in small numbers, are irregularly distributed, and often occur in small groups. They actually are hemosiderin granules that can be confirmed with ferricyanide iron stains. They are found after splenectomy, in some patients with sideroblastic anemias, and occasionally in patients with severe hemolytic anemia.
Three types of RBC inclusions cannot be seen with Wright’s or Giemsa stain. All three require supravital staining techniques or other special procedures. Reticulocytes (discussed in detail later) are the stage in RBC maturation just before full maturity. Their number serves as an index of bone marrow RBC production. Hemoglobin H inclusions can sometimes be seen on a reticulocyte preparation as uniformly distributed small round dots somewhat resembling basophilic stippling but of slightly differing sizes. If a reticulocyte is affected, the Hb H inclusions coexist with the more irregular and more linear reticulum structures. Heinz bodies also require a special staining procedure and may need RBC pretreatment with a strong oxidizing agent such as phenylhydrazine. Heinz body formation is most often found in anemias due to RBC enzyme defects, “unstable” hemoglobins (Chapter 5), and certain uncommon hemoglobins such as hemoglobin Koln and Zurich. The Heinz bodies are small, scattered, dotlike structures of varying size in the RBC derived from denatured hemoglobin.
Limitations of the Peripheral Blood Smear Examination
The peripheral smear is one of the most useful laboratory procedures in hematology. There obviously are many limitations; for example, a peripheral smear cannot demonstrate the presence of anemia per se, which must be detected by means of either the Hb level, Hct value, or RBC count. Also, many etiologies of anemia are associated with nonspecific peripheral blood changes. In some cases in which the peripheral smear is highly suggestive, it may not be so in early stages of the disease. Even if characteristic cell changes are present, there may be different underlying causes for the same morphologic type of anemia, different causes that call for different treatment. Finally, some conditions produce anemia without any demonstrable morphologic changes in the RBC on the peripheral smear. The same comments about RBCs are also generally applicable to the WBCs of the peripheral smear. However, it is often possible to predict leukocytosis by comparing the overall visual ratio of WBCs to RBCs. A differential count of the various WBC forms is done from the peripheral smear.