An international collaborative study on the performance of protein C antigen assays. Report of the ICTH subcommittee on protein C. International Committee on Thrombosis and Haemostasis.

An international collaborative study was undertaken to evaluate the performance and specificity of protein C antigen (PC) assays. Thirteen lyophilized plasma samples were distributed among 17 laboratories and analysed with 24 methods. No statistically significant results were obtained with the different methods in plasmas containing only the protein C zymogen. ELISA's, RIA's and IRMA's were found to be more sensitive than the electro-immunoassay. In plasmas of patients on oral anticoagulant treatment ELISA methods tend to give lower PC antigen levels than the electro-immunoassay. Complexes between activated protein C (APC) and the protein C inhibitor (PCI), when present in plasma together with PC zymogen, are detected with 100% efficiency in the electro-immunoassay, with 50% efficiency in the ELISA, and with less than 10% efficiency in the RIA or in assays using monoclonal antibodies against PC. Mean coefficient of variation was calculated to be 22%, and could be reduced - especially in case of the ELISA by normalisation. Within laboratory variation was calculated to be 11.7% and between laboratory variation 17.8%.