In the Chromogenix Coamatic® Protein C assay, what substances could interfere with the assay, how will they affect results, and what can be done to overcome the interference?

A low protein C activity is expected in aprotinin treated patients because aprotinin is an inhibitor of activated protein C. Oral anticoagulant therapy interferes with the formation of g-carboxyglutamic acid moiety of the protein C molecules during biosynthesis in the liver, which results in a loss of anticoagulant activity. Non-carboxylated forms of protein C molecules that are inactive in vivo can still be activated by snake venom or thrombin-thrombomodulin and retain amidolytic activity in vitro. Assays using chromogenic substrates will therefore over-estimate the true level of protein C activity in plasma from patients receiving OAC’s. Streptokinase also influences the hydrolysis of S-2366™. Sample blank activities should be determined with plasma from patients with thrombolytic disorder treated with streptokinase, as well as plasma where contact activation is suspected, should be compared to sample blank activities. A high blank activity may indicate contact activation has occurred. S-2366™ is also sensitive to thrombin. This interference can be quenched by the addition of a thrombin inhibitor such as I-2581.