Effect of hydroxyurea on immature reticulocyte fraction in sickle cell anemia.

Immature reticulocyte fraction (IRF) is a good indicator of bone marrow erythropoiesis in response to hemolysis or tissue hypoxia and is markedly increased in sickle cell disease (SS). We compared IRF changes in SS patients with those who were treated with hydroxyurea (SS-HU), and those who had concurrent alpha globin gene deletion (SS-(- deletion). Forty-two patients including 16 SS, 16 SS-HU, and 10 SS-alpha-deletion patients were studied. Significant decreases (P <.01) in reticulocyte indices including IRF, the reticulocyte percentage, and absolute reticulocyte count (ARC) were observed in SS-alpha-deletion compared to SS patients. On the other hand, although the reticulocyte percentage (P <.01) and ARC (P <.01) were significantly decreased in SS-HU compared with SS patients, the IRF was persistently elevated in both groups (P = .4), suggesting continuous bone marrow stimulation in SS-HU patients in response to tissue hypoxia. The possible underlying physiological mechanisms are discussed.