Analytical and clinical evaluation of commercial protein C assays

Protein C deficiency is an important risk factor for thrombosis. Recently several commercial assays for this protein have become available. We evaluated two antigen and three activity assays in 50 healthy volunteers, 52 patients with deep venous thrombosis (DVT) proven by positive impedance plethysmography (IPG) and flebography, and 48 control patients referred to us for objective diagnosis of DVT but with repeatedly negative IPG. All subjects were less than 50 yr of age. Based upon assay performance, correlation of the assays, influence of heparin, and practical ease, an ELISA antigen assay and a chromogenic activity assay are recommended. The prevalence of protein C deficiency cannot be accurately determined from this study, because 3 DVT negative and 13 DVT positive patients were receiving coumarin therapy. However, based upon protein C:factor II antigen ratios, at least 7 of the DVT positive patients are likely to be protein C-deficient. Protein C determination should, therefore, be considered in all DVT patients less than 50 yr of age.