Reticulocytes and reticulocyte enumeration

Reticulocytes are immature red blood cells (RBCs). Reticulocytes originate in the bone marrow from the orthochromatic normoblast through nuclear exclusion. They are released into the peripheral blood after a period of maturation in the bone marrow and undergo further differentiation into mature RBCs. The enumeration of peripheral blood reticulocytes (“reticulocyte counting”) is often performed to obtain information about the functional integrity of the bone marrow since the reticulocyte count reflects the erythropoietic activity of the bone marrow, the rate of reticulocyte delivery from the bone marrow into the peripheral blood, and the rate of reticulocyte maturation. Reticulocytosis (an increased number of peripheral blood reticulocytes) occurs in anemic patients with a functional bone marrow, whereas anemic patients with a dysfunctional bone marrow produce decreased numbers of reticulocytes, and have decreased numbers of peripheral blood reticulocytes (i.e., reticulocytopenia). In addition to the evaluation of anemic patients, reticulocyte enumeration is also of value in monitoring bone marrow regenerative activity after chemotherapy or bone marrow transplantation. In the laboratory, the differentiation of the reticulocyte from the mature RBC is based on the presence of RNA and other substances in the reticulocyte, which are lost during differentiation into the mature RBC. Manual counting of reticulocytes by light microscopy with supravital dyes for RNA was developed in the 1940s and remains the standard method of reticulocyte enumeration. However, automated methods of reticulocyte enumeration developed during the past decade are much more accurate, precise, and cost-effective than manual counting, and are increasingly being performed in the clinical laboratory. In addition, the newer techniques provide a variety of reticulocyte-related parameters, such as the reticulocyte maturation index and immature reticulocyte fraction, which are not available with light microscopy. These new parameters are under evaluation in the clinical diagnosis and monitoring of anemia and other diseases. This article reviews the physiology and pathophysiology of the reticulocyte, laboratory techniques of reticulocyte enumeration, and the clinical applications of reticulocyte counting. Reticulocyte enumeration is the subject of several reviews (1–7).