Reticulocytes are immature red blood cells (RBCs), typically composing about 1% of the red cells in the human body. They are produced in the bone marrow when stem cells differentiate and progress toward RBC development, eventually forming reticulocytes and finally mature RBCs after circulating for about a day in the blood stream. This test measures the number and percentage of reticulocytes in the blood and serves as an indicator of the adequacy of bone marrow red blood cell (RBC) production. The reticulocyte count is used to help determine if the bone marrow is responding adequately to the body’s need for red blood cells (RBCs) and to help determine the cause of and classify different types of anemia. INCREASED reticulocyte count is associated with bleeding, erythroblastosis fetalis, hemolytic anemia, and kidney disease with increased erythropoietin production. DECREASED in iron deficiency anemia, pernicious anemia or folic acid deficiency, aplastic anemia, radiation therapy, and bone marrow failure caused by infection or cancer.