In patients with intense erythropoietic stimulation, “young” basophilic reticulocytes are released prematurely from the bone marrow into the peripheral blood, causing shortened reticulocyte maturation time in the bone marrow (sometimes to < 1 day), but a longer reticulocyte maturation time in the peripheral blood. The RPI is a corrected reticulocyte count that accounts for reticulocytes in all development stages.
Reticulocyte production (maturation) index = reticulocyte count [%] × patient’s packed cell volume/0.45) / maturation time in peripheral blood.
Maturation time (days) varies according to Hct:
Reticulocyte production index of > 3 suggests normal bone marrow response to anemia while < 2 suggests inadequate response.
See calculator.