Objectives

List important diagnoses that can be suggested by visual abnormalities of red blood cells (RBCs), WBCs, and platelets on the peripheral smear.

The peripheral blood smear is the often-neglected foundation of the evaluation of a patient with any cytopenia or blood dyscrasia. One would not think of evaluating a patient with chest pain without looking at the electrocardiogram or a patient with cough and fever without looking at the chest radiograph. With hematologic abnormalities there is too often a reliance on numbers and automated results.

This chapter will familiarize the reader with the blood smear in conditions where it is particularly useful. It will provide an understanding of the normal appearance of the different types of WBCs as well as the normal and important abnormal appearances of RBCs. As demonstrated, the ability to perform a quantitative evaluation of platelets on the peripheral smear is an important skill in evaluating the patient with thrombocytopenia.

While detailed and complex analysis of blood smears is the role of the hematologist, a fundamental knowledge of the use of this diagnostic tool will benefit any clinician.

Specific to the evaluation of WBCs, a normal peripheral smear should contain a spectrum of mature leukocytes, including lymphocytes, monocytes, neutrophils, eosinophils, and basophils.

Lymphocytes constitute 20% to 25% of the total circulating leukocytes. They are characterized by a clumped acentrically located nucleus and thin rim of blue cytoplasm. Lymphocytes are larger than erythrocytes and are typically 8 to 10 µm in diameter. Atypical lymphocytes are differentiated by a larger cytoplasm. They are often observed in the setting of viral infections (e.g., infectious mononucleosis).