INTRODUCTION

Examination of the peripheral blood smear is an inexpensive but powerful diagnostic tool in both children and adults. It provides rapid, reliable access to information about a variety of hematologic disorders. The smear offers a window into the functional status of the bone marrow, the factory producing all blood elements. It is particularly important when assessing cytopenic states (eg, anemia, leukopenia, thrombocytopenia). Review of the smear is an important adjunct to other clinical data; in some cases, the peripheral smear alone is sufficient to establish a diagnosis [1].

Automated machines that deliver increasingly sophisticated data about blood counts and morphology tend to generalize and include a wide array of morphologic abnormalities. However, only an experienced reviewer can weigh the relative significance of observed findings and assess their importance within the context of other clinical data. A trained eye will also appreciate other subtleties of morphology that may be undetected by automated review. (See "Automated hematology instrumentation".)

Review of the peripheral smear is not required in all patients with a hematological disorder. Certain straightforward conditions such as iron deficiency anemia can be easily diagnosed on the basis of clinical information and basic laboratory data (eg, mean corpuscular volume, serum iron, ferritin) alone. However, there are a number of settings in which interpretation of the peripheral smear is especially important. Three examples include:

• Hemolytic anemia — Review of red cell morphology may identify the cause of erythrocyte destruction (eg, the presence of bite cells points to a Heinz body hemolytic anemia) and the ultimate diagnosis (eg, oxidant damage to the red cell secondary to drugs)
• Thrombocytopenia — Distinguishing between increased platelet consumption and reduced platelet production can often be made through review of platelet size
• White cell disorders — The precise disease classification may rely upon evaluation of abnormal circulating cells (eg, the presence of Auer rods in a blast form in patients with acute myeloid leukemia)

Evaluation of the peripheral blood smear will be discussed here. Evaluation of bone marrow aspirate smears is discussed separately. (See "Evaluation of bone marrow aspirate smears".)