increased erythropoiesis in the bone marrow Chronic hemolytic disorders, such as sickle cell anemia or hereditary spherocytosis, may first be detected during an aplastic crisis when the reticulocyte count is low Unless the underlying disorder is recognized, the physician may be misled by this finding Furthermore, because the reticulocyte count is expressed as a percentage of total red cells, it must be indexed for the degree of anemia The easiest way to calculate the reticulocyte index is to multiply the reticulocyte count by the reported hemoglobin or hematocrit (HCT pt [patient]) divided by normal hemoglobin or hematocrit (HCT nl [normal]): For example, a reticulocyte count of 5% in a child with severe iron-deficiency anemia and a hematocrit of 6% are not elevated when corrected for the degree of anemia (5% × 6%/33% = 0.9%) The normal reticulocyte index is between 1.0 and 2.0 The MCV varies with age, necessitating the use of age-adjusted normal values ( Table 62.4 ) In addition, the measured MCV represents an average value If microcytic and macrocytic red cells are present in the peripheral blood as, for example, in a patient with combined iron deficiency and B12 deficiency, the MCV may remain normal Therefore, the peripheral smear should be examined carefully to determine whether the MCV reflects a single population of red cells of uniform size, or two or more populations of distinctly different size The red cell distribution width (RDW) is elevated in the presence of increased variation in red cell size As shown in Figure 62.1 , the reticulocyte count and MCV help in the initial classification of anemia but leave the physician with broad categories of disease, rather than specific diagnoses In many instances, the history and physical examination, when coupled with these laboratory measurements, permit identification of a particular disorder When this is not possible, consultation with a hematologist, additional laboratory studies, and careful examination of the peripheral smear may be required to characterize the disease