Standardisation of a simple method for the determination of antithrombin activity.

A procedure is described for performing a functional assay of serum antithrombin activity. The method consists of adding serum to a thrombin solution and measuring, after a fixed incubation time, the residual thrombin activity on a substrate plasma. The mean serum antithrombin activity found in 96 healthy blood donors using this procedure was 109.5% (range 82%-160%). The method was linear over an activity range between 30.5% and 176%, was significantly correlated with antithrombin-III protein concentration determined by radial immunodiffusion (r = 0.86, P less than 0.01), and showed good reproducibility (coefficient of variation 2.7%). On account of its simplicity and precision this functional assay should be of considerable use in evaluating hypercoagulability.