AbstractThe fibrinogen clotting time (FCT) is a measure of thrombin activity, and is used to evaluate the potential thrombogenicity of prothrombin complex concentrates (PCC). We have defined end points for clot formation in this test which allow the measurement in PCC of thrombin concentrations as low as 0.001 IU/ml. The FCT of thrombin and PCC samples which did not contain antithrombin III (ATIII) were the same when measured at 20°C or 37°C. In the presence of ATIII (0.05 or 0.25 IU/ml), samples of PCC which were known to contain thrombin showed shorter FCT at 20°C than at 37°C. Inclusion of both ATIII (0.25 IU/ml) and heparin (4 IU/ml) in PCC ensured the complete inactivation of endogenous throm