Protein C: comparison of different assays in normal and abnormal plasma samples

Two assay methods for Protein C (PC) are described. Both tests are based on the activation of PC by a fraction of the venom of the Copperhead snake. Activated PC can be measured either by cleaving of a chromogenic substrate or by an APTT test. Both tests were compared with ELISA tests for PC. In 27 healthy persons the results were around 100% by all test systems when compared with normal pooled plasma. In 13 patients under stable anticoagulant therapy the average results were 54% for the ELISA and 52% for the chromogenic substrate test whilst the corresponding figures were 22% for the APTT test and 25% for the Quick test. In 4 cases of liver cirrhosis PC was diminished and there was no difference between the three tests. In two patients with congenital PC deficiency all results were close to 50% and in three patients the ELISA and the substrate tests were normal but the APTT test showed results of 44, 28 and 54% respectively. These results give rise to the conclusion that the snake venom activator is also capable of activation of decarboxylated PC molecules which can thereby split a chromogenic substrate but remain inactive in an APTT test.