Protein C antigen is not an acute phase reactant and is often high in ischemic heart disease and diabetes.

Protein C, an antithrombotic protein, was measured immunologically in 299 patients with clinical conditions associated with a high frequency of venous or arterial thromboembolism. The mean protein C antigen (PC:Ag) level was high for 48 patients with ischemic heart disease and, to a lesser extent, for 95 diabetics. In 28 patients with thrombotic strokes, 48 patients with proximal deep-vein thrombosis and in 80 patients with localized or metastatic tumors, mean PC:Ag was normal. Comparison of the pattern of changes of PC:Ag levels with those of fibrinogen, orosomucoid and prothrombin in 21 patients during the postoperative period and in 20 patients with active rheumatoid arthritis ruled out the possibility that high PC:Ag is non-specific, acute-phase reaction to inflammation, tissue injury or neoplastic growth. Therefore, high PC:Ag might be specifically related to the thrombotic tendency of these patients, but the mechanism of such a relationship remains to be clarified.