title: A sensitive, accurate assay for extrinsic pathway inhibitor (EPI) activity in rabbit plasma: paradoxical effect of excess exogenous factor X.
abstract: A sensitive assay is described for the measurement of rabbit plasma EPI activity in experimental studies of induced hypercoagulable states in this species. It is based upon the ability of a dilution of rabbit plasma to inhibit human factor VIIa/rabbit brain tissue factor (TF) catalyzed activation of human factor IX (tritiated activation peptide release assay). Addition of 3H-factor IX to the reaction mixture is delayed for 45 minutes to allow full inhibition by EPI/factor Xa complex before the residual catalytic activity of factor VIIa/TF is measured. Although the diluted rabbit plasma test sample contains both EPI and factor X, supplemental factor X is added to the reaction mixture to assure that only EPI content of the test sample affects the assay result. However, the final concentration of factor X in the reaction mixture is critical. Too high a concentration of factor X diminishes the sensitivity of the assay. The reason for this phenomenon, which was observed with both human and rabbit factor X preparations, is unknown.
authors.affiliation: Department of Medicine, University of California, San Diego 92093.
authors.forename: T A
authors.initials: TA
authors.lastname: Warr
authors.forename: L V
authors.initials: LV
authors.lastname: Rao
authors.forename: S I
authors.initials: SI
authors.lastname: Rapaport
mesh_terms: D000818:Animals; D005164:Factor IX / Q000494:pharmacology; D005167:Factor VII / Q000037:antagonists & inhibitors*; D015942:Factor VIIa / Q000037:antagonists & inhibitors; D005170:Factor X / Q000032:analysis / Q000494:pharmacology*; D008074:Lipoproteins / Q000097:blood*; D011480:Protease Inhibitors / Q000097:blood*; D011817:Rabbits; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013925:Thromboplastin / Q000037:antagonists & inhibitors*; D014316:Tritium
chemical_list: D008074:Lipoproteins; D011480:Protease Inhibitors; C051928:lipoprotein-associated coagulation inhibitor; D014316:Tritium; D005167:Factor VII; D005164:Factor IX; D005170:Factor X; D013925:Thromboplastin; D015942:Factor VIIa