Time course of Hb level and sEpo (upper panel) and of sTfR, reticulocyte count and HFR (lower panel) in a patient with megaloblastic anemia due to folate deficiency treated with folic acid (15 mg per day IM). A marked decrease in serum Epo was seen after the first injection and before any increase in Hb level. There was a parallel increase in HFR, sTfR, and reticulocyte count, indicating a rapid expansion of the erythroid marrow during the first days of treatment.