Blood morphology adds greatly to the value of a routine blood count. A skillful examination of a well-made blood smear constitutes the most valuable single procedure in the hematology laboratory. In spite of normal blood count figures, careful observation of blood morphology suggested previously unsuspected disorders. For example, in some cases, the finding of hypersegmented neutrophils on the blood smear was the first hematologic clue to a significant deficiency of vitamin B12 or folate, the erythrocytes lacking the characteristic macrocytosis associated with such deficiencies.
The reporting of blood morphology has been improving in recent years, although in many clinical laboratories, it still receives little attention. The blood count report form frequently leaves only a tiny area for morphologic comments!
A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and examined under a microscope by a trained laboratorian. It is a snapshot of the cells that are present in the fluid portion of the blood (plasma) at the time the sample is obtained. The results of a blood smear typically include a description of the appearance of the red blood cells, white blood cells, and platelets as well as any abnormalities that may be seen on the slide.
Red Blood Cell Morphology
Red blood cells (erythrocytes) are biconcave disks with a diameter of 7-8 microns, which is similar to the size of the nucleus of a resting lymphocyte. In normal red blood cells, there is an area of central pallor that measures approximately 1/3 the diameter of the cell. Though reference ranges vary between laboratories and in different age groups, normocytic red blood cells typically have a mean corpuscular volume (MCV) between 80-100 fL.
Spherocytes are formed when there is a loss of part of the red blood cell membrane. This may occur in the setting of immune-mediated hemolysis or congenital red cell membrane defects such as hereditary spherocytosis. Spherocytes are smaller than normal red blood cells and lack central pallor. They are less deformable and less able to navigate through small vessels, leading to increased destruction in the spleen.
Microcytic red blood cells measure 6 microns or less in diameter. The mean corpuscular volume is generally less than 80 fL, though the normal range varies slightly between laboratories and in different age groups. In contrast to spherocytes, which are also decreased in diameter, microcytes retain their central pallor. In microcytosis due to iron deficiency, the central pallor is increased (more than 1/3 the diameter of the cell).
Teardrop cells in a peripheral blood smear from a patient whose bone marrow was extensively replaced by B lymphoblastic leukemia. Teardrop cells may be seen in the setting of marrow infiltration (by fibrosis, granulomatous inflammation, hematologic or metastatic malignancy), splenic abnormalities, megaloblastic anemia, and thalassemia. True teardrop cells have slightly rounded or blunted ends. In contrast, teardrop cells that are formed as an artifact of smear preparation have very sharp points, all facing in the same direction.
Cabot rings are thin, threadlike, red to violet rings or “figure 8” shaped inclusions in red blood cells. Cabot rings are remnants of the mitotic spindle, and can be seen in megaloblastic anemia, medication effect, myelodysplasia and other forms of dyserythropoiesis. In this image of a blood smear from a patient with vitamin B12 deficiency, the Cabot ring is visible as a faint ring-shaped inclusion in the polychromatophilic cell in the center of the field.
Peripheral blood smear of a 38-year-old female with long-standing Crohn’s disease (CD) and development of microcytic anemia. The smear shows numerous target cells and a spur cell (top right). All liver function tests were abnormal indicating that the target cells are due to liver disease secondary to CD. This patient originally had a concomitant iron deficiency. Spur red cells have elongated projections while Burr cells are red cells with circumferential blunted borders. The former is typically seen in liver disease while the latter is seen in uremia. The “Burr” morphology, in this case, is artifactual related to slide preparation and not related to uremia.
Acanthocytes in two patients with liver disease. Acanthocytes (also called spur cells) are spiculated cells with irregular, pointed or clublike projections that are unevenly distributed on the cell surface. Central pallor is absent. Acanthocytes form as a result of membrane lipid abnormalities, and can be seen in liver disease, neuroacanthocytosis, severe malnutrition, and abetalipoproteinemia.
Sickle cells (drepanocytes) are elongated red blood cells with pointed ends. They are seen in sickling hemoglobinopathies such as sickle cell anemia (homozygous hemoglobin SS), hemoglobin SD disease, and hemoglobin S/beta-thalassemia.
Echinocytes (Burr Cells) have multiple short, blunt projections evenly spaced over the cell surface. The central pallor is retained. Echinocytes can be seen in uremic patients. They can also be seen as an artifact of slide preparation or prolonged specimen storage.
Stomatocytes are red cells with a slit-like or “fish-mouth” central pallor. Stomatocytes may be seen in patients with alcoholic liver disease, hereditary stomatocytosis, or Rh null disease, among other conditions. They may form in vitro in the presence of certain cationic medications or low pH.
Red cell fragments (schistocytes) in a patient with microangiopathic hemolysis due to thrombotic thrombocytopenic purpura (TTP). Small triangulocytes and larger, crescent-shaped helmet cells are present. Both of these are red cell fragments and would be included in the schistocyte count. When numerous small schistocytes are present, automated cell counters may count the small red cell fragments as platelets, leading to a falsely elevated automated platelet count.
Oxidative hemolysis induced by furosemide in a patient with G6PD deficiency. In oxidative hemolysis, the peripheral smear may show irregularly contracted red blood cells, hemighost or blister cells, and spherocytes. Irregularly contracted cells lack central pallor, and the hemoglobin appears condensed and irregularly distributed in the red blood cell.
Clumping (agglutination) of red blood cells is frequently caused by cold agglutinins. Cold agglutinins are IgM antibodies that may arise following viral or Mycoplasma infections, or in the setting of plasma cell or lymphoid neoplasms. Agglutination of red cells can interfere with red blood cell indices. The red blood cell count may be falsely decreased, and the MCV falsely increased, as clumps of red cells are measured as single cells. The hemoglobin level will be accurate, as this parameter is measured after lysing the red cells.
Polychromasia (polychromatophilic cells) in a neonate. Polychromatophilic cells are young red blood cells that have been recently released from the bone marrow. They are larger than mature red cells, and are bluish in color. Polychromasia is increased in hemolysis, blood loss, and marrow infiltration. Normal neonates have a higher number of polychromatophilic cells than older children and adults.
Howell-Jolly body: the red blood cell in the center of the image contains a Howell-Jolly body. Howell-Jolly bodies are small (0.5-1 micron) purple inclusions that contain DNA. They are thought to represent chromosomes that have separated from the mitotic spindle that is left behind when the red cell nucleus is extruded. These inclusions are generally removed by the spleen. Patients with asplenia or hyposplenism may have increased Howell-Jolly bodies on their peripheral blood smear. A nucleated red blood cell is also present at the bottom left side of the image.
Blood Morphology – Erythroid Precursors
Proerythroblasts (also called pronormoblasts) are the earliest erythroid precursors. These are large cells with basophilic, agranular cytoplasm, round nuclei, and high nuclear-cytoplasmic ratios. The chromatin is evenly dispersed, but is slightly more dense than myeloblast chromatin. One or more nucleoli may be visible. A perinuclear clear area (hof) may also be seen. A single proerythroblast is seen in the center of this image. Polychromatophilic and orthochromic normoblasts are present on the right side of the field.
Basophilic normoblasts (also called basophilic erythroblasts or early erythroblasts) are smaller than proerythroblasts, with more condensed chromatin and lower nuclear-cytoplasmic ratios. The cytoplasm is deep blue, and a pale perinuclear halo may present. The two cells in the center of the field are basophilic normoblasts.
Orthochromic normoblasts (also called orthrochromatophilic normoblasts, orthrochromatophilic erythroblasts, or late erythroblasts) are slightly larger than mature red blood cells. They have small, round nuclei and dense, pyknotic chromatin. The cytoplasm is generally slightly more basophilic than the cytoplasm of a mature red blood cell.
Erythroid precursors at various stages of maturation. Basophilic normoblasts are present at the center of the field. Polychromatophilic normoblasts and orthochromic normoblasts are present near the bottom of the field. As erythroid precursors mature, the cell size and nuclear-cytoplasmic ratio decrease, and the chromatin becomes progressively more condensed. The cytoplasm changes color from deep blue to gray-blue to gray-pink as the hemoglobin content increases.
A deficiency of either vitamin B12 or folic acid results in megaloblastic erythroid cells-megaloblasts. These deficiencies result in a decrease in DNA synthesis which slows and inhibits DNA replication (nuclear division). Nuclear maturation is slowed whereas cytoplasmic maturation (largely dependent on RNA function) is unaffected. The impaired nuclear maturation is seen as open, loose, immature chromatin (cut-salami pattern). In contrast to the nucleus, the cytoplasm of megaloblastic cells is abundant with normal hemoglobinization. This disparity between nucleus and cytoplasm is known as nuclear-cytoplasmic asynchrony. Although most noticeable in erythroid cells failure of DNA synthesis also affects myeloid and megakaryocytes. Giant bands and hypersegmented polymorphonuclear neutrophils are common.
Vacuolated erythroid precursors can be seen in copper deficiency, Pearson syndrome, and myelodysplastic syndromes. In this image, a vacuolated erythroid precursor is adjacent to another dysplastic erythroid precursor with megaloblastic features (nuclear-cytoplasmic asynchrony) and nuclear irregularities. The patient had acute myeloid leukemia with myelodysplasia-related changes.
White Blood Cell Morphology
Mature segmented neutrophils have condensed chromatin and 2 to 5 nuclear lobes separated by thin filaments. The cytoplasm is pale pink and contains numerous specific granules. In healthy adults, approximately 40-80% of peripheral blood leukocytes are segmented neutrophils.
Band neutrophils are slightly less mature than segmented neutrophils and have indented, unsegmented “C” or “S” shaped nuclei. Band neutrophils normally account for approximately 5-10% of peripheral blood leukocytes. An increased proportion of band neutrophils can be seen in infectious and inflammatory conditions.
Toxic granulation in neutrophils is found in inflammatory states. The toxic granules are azurophilic, and may be found in the promyelocyte, metamyelocyte, band (shown here), and mature stages.
Hypersegmented neutrophil in a patient with vitamin B12 deficiency. Hypersegmented neutrophils have 6 or more nuclear lobes. They are typically seen in megaloblastic anemia due to vitamin B12 or folic acid deficiency, but may also be present in myelodysplastic syndromes and rare congenital conditions.
Pelger-Huet anomaly is a congenital autosomal dominant anomaly in which neutrophil nuclei fail to segment normally. In homozygotes, the nucleus is round. In heterozygotes, most granulocytes have bilobed nuclei (“pince-nez” cells) resembling bands.
Neutrophils with prominent hypolobation and Pseudo Pelger-Huet cytomorphology in a patient seen in the posttransplant setting. This patient was on mycophenolate which is sometimes known to induce such changes. One must be cautious not make a diagnosis of myelodysplastic syndrome in the setting.
Neutrophilic metamyelocytes have condensed chromatin and a slightly indented nucleus (the indentation is less than half the diameter of the nucleus). The cytoplasm contains abundant specific granules and rare primary (azurophilic) granules. Metamyelocytes are not normally present in the peripheral blood, but can be seen in infectious or inflammatory states, and in other reactive and neoplastic conditions.
Myelocytes contain both primary (azurophilic) and secondary/specific (pink or lilac) cytoplasmic granules. The proportion of secondary granules increases as the cell matures. The nucleus is round and lacks a nucleolus. The chromatin is more condensed than that of promyelocytes. Myelocytes are not normally present in peripheral blood, but may be seen in infectious/inflammatory conditions, growth factor effect, marrow infiltration, and myeloid neoplasms.
Promyelocytes are larger than myeloblasts, and have basophilic cytoplasm containing primary (azurophilic) granules. A Golgi zone may be visible as a paranuclear hof or clearing. The nuclear chromatin is finely dispersed, and nucleoli may be visible. Promyelocytes comprise approximately 2% of nucleated cells in the bone marrow and do not circulate in peripheral blood under normal conditions.
Myeloblasts are approximately 15-20 microns in size, and have high nuclear to cytoplasmic ratios. The nucleus is usually round to oval, but may be irregular. The chromatin is smooth and open, and one or more nucleoli may be present. The cytoplasm is pale blue. Cytoplasmic granules are not typically seen, though rare small granules may be present. Leukemic myeloblasts may contain few cytoplasmic granules or Auer rods, which are reddish, linear structures composed of fused primary granules. The presence of Auer rods indicates myeloid malignancy.
Eosinophils measure 10-17 microns in diameter, and have abundant, slightly basophilic cytoplasm containing numerous coarse, reddish-orange cytoplasmic granules. Most eosinophils have bilobed nuclei, but occasional forms with trilobed nuclei may be seen. The chromatin is condensed. Eosinophils usually account for a minor subset of peripheral blood leukocytes. Increased numbers of eosinophils can be seen in parasitic infections, allergic conditions, drug hypersensitivity, myeloid neoplasms, and lymphoproliferative disorders.
Basophils have segmented nuclei that are often at least partially obscured by abundant coarse, dark blue to purple cytoplasmic granules. Basophil granules are water soluble, and may wash out during staining. Basophils normally circulate in low numbers. Basophilia may be seen in inflammatory and allergic conditions, hypothyroidism, and myeloproliferative neoplasms.
Normal resting lymphocytes are small cells with condensed chromatin and a small amount of pale basophilic cytoplasm. The nucleus of a resting lymphocyte is just slightly larger than a red blood cell.
Reactive lymphocytes show a range of morphologic features. Reactive lymphocytes with immunoblast-like morphology are large cells with high nuclear-cytoplasmic ratios, condensed chromatin, and deeply basophilic cytoplasm. Another type of reactive lymphocyte has less condensed chromatin and abundant, pale blue cytoplasm that may appear to “hug” adjacent red blood cells. These cells are also called Downey type II cells. They can be seen in a variety of conditions, but are often increased in infectious mononucleosis due to EBV infection.
Lymphocytes with peripherally clumped chromatin and often deep blue cytoplasm similar to plasma cells are termed plasmacytoid lymphocytes. These transitional forms between lymphocytes and plasma cells are seen in the blood of patients with viral infections. These cells are variously known as atypical lymphocytes, lymphocytoid plasma cells or plasmacytoid lymphocytes.
Teresa Scordino, MD. ASH Image Bank: http://www.ashimagebank.org/
Paul C. Hattersley, M.D. and Judith L. Engels, MT: The Reporting of Blood Morphology https://goo.gl/zr26TK
Hattersley, P.G., and Engels, J.L: Neutrophilic Hypersegmentation Without Macrocytic Anemia, West. J. Med. 121:179-184, 1974.
De Gruchy, G.C.: Clinical Hematology in Medical Practice, ed 2, Philadelphia: F.A. Davis Co., 1964, p. 63.
Hattersley, P.G., and Ragusa, D.: Don’t Forget the Morphology: The Importance of Evaluation of Blood Smears, Calif. Med. 103:175-177, 1965.
Megaloblastic Anemia. Department of Pathology, University of Virginia, School of Medicine. https://www.med-ed.virginia.edu/courses/path/innes/rcd/mega.cfm