SIMULTANEOUS ASSAY OF FREE AND TOTAL PROTEIN-S BY ELISA USING MONOCLONAL AND POLYCLONAL ANTIBODIES

The measurement of protein S is essential in the investigation of thro mbophilia. Protein S forms a complex with C4b binding protein which ex ists in equilibrium with the free form but only the free form has anti coagulant cofactor activity. The accurate measurement of protein S has previously proved problematical. We have investigated the use of an E LISA method for protein S, using monoclonal antibodies directed agains t the free form, which can be used in parallel with polyclonal antibod ies for the simultaneous assay of total protein S. The total protein S measurement was based on a well established method while the monoclon al antibody free protein S assay (mAb PS) was less complex than previo us assays with no requirement for prier PEG precipitation and showed i mproved precision (intra-assay CV = 4.8% and 7.1%, between assay CV = 7.1% and 8.6% respectively). The mAb assay compared well with the conv entional PEG precipitation ELISA for free protein S and a functional p rotein S assay based on the prothrombin time, in normal subjects, pati ents with thrombophilia, congenital protein S deficient patients or ou t-patients stabilized on oral anticoagulants, This assay overcomes man y of the problems associated with protein S measurement and is a usefu l tool in the investigation of thrombophilia.