Reticulocytes can be separated from more mature red blood cells based on differences in density. A method for obtaining autologous reticulocytes in ethylenediaminetetraacetic acid (EDTA) whole blood samples containing both autologous and transfused cells uses a microhematocrit centrifuge. The less dense reticulocytes harvested from the top 5 mm of microhematocrit tubes can be used to determine the patient’s phenotype or assess whether a transfusion reaction is taking place. This method can be performed using equipment, reagents, and supplies readily available in most laboratories.