A functional assay for protein C in human plasma.

We describe a functional assay for protein C in human plasma samples based on the ability of activated protein C to prolong the kaolin-cephalin activated partial thromboplastin time of normal plasma. Protein C is separated from its inhibitor by elution of a barium citrate precipitate, and activated by incubation with human alpha-thrombin for one hour. Thrombin is then inhibited by antithrombin III and heparin, heparin neutralized by protamine sulfate, and protein C activity measured in the partial thromboplastin time. 24 normal subjects had a mean protein C level of 94 +/- 12% (SD) of the activity in pooled normal plasma. Seven patients with severe liver disease had a mean protein C of 28%. Eleven patients with disseminated intravascular coagulation had a mean protein C of 29%. Eight patients receiving warfarin therapy had a mean protein C of 17%. The assay is relatively simple and should be suitable for general laboratory use.