Summary
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 <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%.
Keywords
Protein C antigen assay
