Endogenous clearance rate and synthesis of alphafoetoprotein during and after blood exchange transfusion in hyperbilirubinaemic newborns.

Plasma alphafoetoprotein (AFP) concentration was monitored in 22 hyperbilirubinaemic infants during blood exchange transfusion at the age of 0-4 days, in order to determine the endogenous clearance rate and demonstrate possible postnatal synthesis of AFP. The rapid fall in plasma AFP during the transfusion to 19 +/- 4% (mean +/- S.D.) was followed by an increase to 81 +/- 15% of the initial concentration; only a 68% rise was expected after the establishment of a new equilibrium between the intra- and extravascular pools of AFP. Repeated transfusion in the same individuals gave similar findings. In two infants the total amount of AFP in these pools increased by 4.0-7.4 mg after the transfusions. The results could be accounted for by postnatal AFP synthesis. Because of the diffusion of AFP from the extravascular compartment to the intravascular pool the actual turnover rate of AFP could not be measured. The estimated magnitude of endogenous turnover rate of AFP, 0.02-0.09 min-1, gave a mean half-life of about 20 min, which is distinctly shorter than that of AFP in plasma (3-5 days); this could be due to either continuous synthesis of AFP or transfer of presynthesized AFP from the liver cells to extracellular pool. We postulate that either the synthesis or the turnover rate of AFP is controlled by the plasma AFP concentration via a negative feedback mechanism.