The activation of prothrombin complex concentrates by calcium in vitro

The activation of prothrombin concentrate mixtures incubated "in vitro" was investigated. "Activation" was assessed by the correcting effect of suct mixtures on the APTT of a test plasma containing a factor VIII inhibitor and also by the generation of free thrombin. Thrombin itself was found to have insignificant activating effect. Prothrombin concentrates incubated with near-physiological levels of calcium appeared to correct the abnormal APTT to an increasing degree as the calcium concentration was increased. Free thrombin was generated with calcium cencentrations greater than those required for APTT correction. The activation process could be stopped at any stage by adding citrate which removed ionized calcium from the incubation mixture.