Yield of reticulocyte counts and blood smears in term infants

Reticulocyte counts and blood smears are commonly recommended to evaluate jaundice in newborns. To investigate the results and diagnostic yield of these tests, we reviewed a computerized database and medical records of term newborns who had reticulocyte counts (n = 799) or blood smears (n = 781) within the first week after birth at two hospitals. Nearly a threefold difference was noted in reticulocyte counts between the two hospitals (median 8.0% vs 2.8%; P < .0001), apparently due to differences in laboratory methods. Among the patients with "abnormal" reticulocyte counts or blood smears (n = 192), isoimmunization was diagnosed in 54, presumed hemolysis of unknown etiology in two, G6PD deficiency in one, and pyropoikilocytosis in one. We conclude that better standardization of reticulocyte counts is needed. When ordered as screening tests for hemolysis in jaundiced infants, reticulocyte counts and blood smears seldom lead to diagnoses of hemolysis other than isoimmunization.