MEASUREMENT OF TISSUE FACTOR PATHWAY INHIBITOR IN NORMAL AND POSTHEPARIN PLASMA

Assay methods for the detection of both tissue factor pathway inhibito r (TFPI) function (two-stage chromogenic assay) and for TFPI antigen l evels (competitive ELISA) have been developed and applied to the measu rement of TFPI in normal plasma, in post-heparin plasma and to recombi nant TFPI. There was good correlation in TFPI levels, measured using t he two methods (r = 0.848; P < 0.001) in the normal plasma samples (n = 21) with the values ranging from 0.6 to 1.4 units per ml relative to a normal reference plasma pool(assigned 1.0 unit per ml). The post-he parin plasma samples were associated with increased levels of both TFP I functional activity and antigen. However, there was poor correlation between the two methods, with an increase in antigen levels greatly e xceeding the increase in functional activity. This discrepancy was als o found with recombinant TFPI and may reflect the different responses of the two assay methods to lipoprotein-bound TFPI (in the normal plas ma reference) and the 'free' TFPI in post-heparin plasma and recombina nt TFPI. These findings have implications in the choice of suitable re ference materials for the assay of TFPI.