Summary
A simple quantitative method for the determination of antithrombin III in plasma is described. It is based on the preferential precipitation of plasma antithrombin activities by a chemical compound 2,5-diamino-7-ethoxyacridin lactate (rivanol). When the incubation time of plasma with rivanol took place within 0.5 and 2 hours under specifically controlled experimental conditions, rivanol precipitated α2-macro-globulin selectively and quantitatively without any effect on antithrombin III. In the presence of rivanol, the inhibition of thrombin by plasma antithrombin III followed a second-order reaction with a rate constant of 80 1/M/sec. In this assay procedure, only the initial thrombin activity and the 30 minutes residual thrombin activity are required to evaluate the amount of antithrombin III present in the plasma quantitatively. “With proper routine laboratory care, this assay system was found to be accurate and reproducible.