The anemia of prematurity. Factors governing the erythropoietin response.

We performed sequential studies in 45 premature infants (birth weights less than 1500 g) from 7 to 120 days of age to determine factors governing the erythropoietin response to a declining hemoglobin concentration. The hemoglobin level and the plasma erythropoietin showed a significant inverse correlation (r = 0.50, P less than 0.001), as did, even more strikingly, the plasma erythropoietin response and the infants' oxygen-unloading capacity (r = 0.55, P less than 0.001). In infants with "right-shifted" oxygen-hemoglobin dissociation curves (hemoglobin F less than 30 per cent) hemoglobin levels fell 2 to 3 g per deciliter lower than those in infants with "left-shifted" curves (hemoglobin F greater than 60 per cent) before comparable erythropoietin responses occurred. It appears that premature infants respond appropriately to alterations in oxygen unloading capacity and that the position of the oxygen-hemoglobin dissociation curve and not the hemoglobin concentration alone has a major role in modulated erythropoiesis.