Plasma elimination of antithrombin III (heparin cofactor activity) is accelerated in term newborn infants.

Antithrombin III (AT III) levels are markedly increased in newborn infants following exchange transfusion with adult blood, and subsequently return to pre-exchange values. This transient rise in AT III (heparin cofactor activity), was used to estimate its plasma elimination half-life. AT III activities were measured serially, before and after double-volume exchange transfusions with heparinised blood in newborn infants requiring therapy for severe hyperbilirubinaemia. The plasma elimination half-life of AT III activity was calculated to be 3.9 +/- 1.4 h (mean +/- SEM). Compared with published data on the kinetics of AT III infusions in adults, the neonate has a considerably accelerated turnover. This finding has important implications for the design of future therapeutic trials of AT III concentrates and provides further evidence that plasma proteins, including components of the coagulation system, appear to have different kinetics in the neonatal period.