A sensitive immunochemical assay for measuring the concentration of the activated protein C-protein C inhibitor complex in plasma: use of a catcher antibody specific for the complexed/cleaved form of the inhibitor.

Activated protein C (APC) is a serine proteinase that regulates blood coagulation. In plasma it is inhibited mainly by the protein C inhibitor (PCI). The plasma concentrations of APC-PCI complex is increased in hypercoagulative states such as deep venous thrombosis. Formation of the APC-PCI complex induces a drastic conformational change in PCI that exposes new epitopes (neoepitopes) on the molecule. We have devised a simple immunofluorometric sandwich assay for measurements of the concentrations of APC-PCI complex, employing as the catcher, a monoclonal antibody that has a high affinity (K(D) = 4 x 10(-11) M) for a complexation-specific neoepitope that is expressed on PCI. A monoclonal antibody against protein C is employed as the tracer. The method gives a linear dose-response curve (0.06-50 microg/l), has a low detection limit (0.06 microg/l) and no crossreactivity with native PCI at physiologic plasma concentrations. We have now determined the concentration of the APC-PCI complex in healthy individuals.