Relationship between antithrombin and thrombin levels in plasma and serum

Antithrombin levels were measured following the addition of varying amounts of thrombin to defibrinated plasma and following the intravenous injection of thrombin into rats. In the in vitro studies the decrease in antithrombin was proportional to the thrombin added. The addition of 30–40 u of thrombin to 1 ml of plasma caused a 50% decrease in antithrombin. Similar results were obtained in the in vivo studies. Six-hundred units of thrombin injected into heparinized rats caused a 40% reduction in plasma antithrombin. Thrombin generation in samples of recalcified plasma was measured according to the method of Pitney and Dacey ( J. Clin. Path. 6:9, 1953) and the values compared with changes in antithrombin. The amount of thrombin generated when full strength plasma was recalcified was only a fraction of the amount potentially available according to estimations by the two-stage method. When plasma was diluted with one to two volumes of saline before activation the amount of thrombin generated was increased two- to threefold. The antithrombin level of the serum from the diluted samples was correspondingly decreased.