Peripheral Blood Smear

Excerpt

Examination of the peripheral blood smear should be considered, along with review of the results of peripheral blood counts and red blood cell indices, an essential component of the initial evaluation of all patients with hematologic disorders. The examination of blood films stained with Wright's stain frequently provides important clues in the diagnosis of anemias and various disorders of leukocytes and platelets.

Normal human red blood cells are biconcave disks (diskocytes) with a mean diameter of about 7.5 μm. Erythrocytes are slightly smaller than small lymphocytes. The hemoglobin of red cells is located peripherally, leaving an area of central pallor equal to approximately 30 to 45% of the diameter of the cells. Cells of normal size and hemoglobin content (color) are termed normocytic and normochromic. Larger than normal erythrocytes are macrocytes (diameter greater than 9 μm); small red cells are microcytes (diameter less than 6 μm); and those with central pallor greater than 50% of the diameter are hypochromic. Abnormal variability in size is termed anisocytosis; unusual variation in shape is called poikilocytosis; and significant differences among erythrocytes in the amount of central pallor is referred to as anisochromia. Polychromatophilia means the erythrocytes have a blue-gray hue to the color of their cytoplasm.

From a diagnostic standpoint, poikilocytosis has no specificity, but the recognition of specific forms of poikilocytes (irregularly shaped cells) often points to specific disorders. Spherocytes are round, densely staining red cells that lack central pallor and have a smaller than normal diameter. In stomatocytes, the area of central pallor is elliptical rather than round, giving the cell the appearance of the opening of a mouth (stoma). Target cells (codocytes) have a centrally located disk of hemoglobin surrounded by an area of pallor with an outer rim of hemoglobin adjacent to the cell membrane giving the cell the appearance of a target. Leptocytes (or wafer cells) are thin, flat cells with the hemoglobin at the periphery of the cell. Sickle cells (drepanocytes) are elongated, sometimes crescent-shaped, erythrocytes with pointed ends. Elliptocytes (ovalocytes) range from slightly oval to elongated cigar-shaped forms. Teardrop erythrocytes (dacryocytes) are red cells with one end round and the other end more pointed. Acanthocytes have several (usually 3 to 7) irregularly spaced blunted projections from the margin of the cells. Echinocytes are also cells with cytoplasmic projections, but in contrast to acanthocytes, the projections are typically evenly spaced on the cell surface, more numerous (often 10 to 15), and frequently have sharper points. Schizocytes (schistocytes) are fragmented erythrocytes appearing in a variety of morphologic forms such as small triangular erythrocytes, helmet cells, and normal-size erythrocytes with 2 to 3 pointed surface projections (keratocytes, or "horn cells"). Round erythrocytes with a single, elliptical or round surface defect are termed bite cells. Rouleaux formation is a phrase denoting the stacking of erythrocytes, generally in a curving pattern.

Morphologic identification of inclusion bodies within erythrocytes can be helpful clinically. Howell–Jolly bodies are purple spheres, usually about 0.5 μm in diameter, presenting singly, or rarely multiply, in the cytoplasm. Basophilic stippling of erythrocytes refers to numerous very small coarse or fine blue granules within the cytoplasm. When the stippled particles are due to iron granules (demonstrable by the Prussian blue stain), they are termed Pappenheimer bodies. Malaria parasites may appear as cytoplasmic inclusion bodies within erythrocytes. Platelets overlying erythrocytes may be mistaken for erythrocyte inclusions.

There are a number of important morphologic abnormalities of mature granulocytes. Cytoplasmic vacuoles may be recognized. Toxic granulation refers to small, dark blue-staining granules. Döhle bodies are light blue cytoplasmic inclusions, 1 to 2 μm in diameter. The Pelger–Huët anomaly, a disorder characterized by impaired nuclear segmentation of mature neutrophilic granulocytes, appears morphologically as cells with bilobed nuclei (dumbbell or eyeglass shapes) or with round or oval nuclei (Stodtmeister cells). Hypersegmented neutrophils are cells in which there are six or more nuclear lobes.

Reactive lymphocytes are usually larger than small lymphocytes, may have cytoplasmic vacuolization, sometimes have deep blue staining of the periphery of the cytoplasm, and contain nuclei that may be kidney bean or monocytoid in shape.

Most platelets in the peripheral blood have diameters between 1 and 3 μm. Platelets greater than 3 μm in diameter are "large" (megathrombocytes). In a normal person usually less than 5% of the platelets appear large.

Figure 155.1 shows examples of morphologically normal and abnormal erythrocytes.