title: Antifactor Xa activity measured with amidolytic methods.
abstract: Methods for the assay of antifactor Xa activity in the presence and absence of heparin are described. Diluted plasma is incubated with bovine, activated factor X (Xa) in stage I, and remaining Xa is measured with the chromogene substrate Bz-Ile-Glu-Gly-Arg-pNA in stage II. In the presence of heparin, the inactivation is completed in 30 sec, and this method measures total Xa-inactivating capacity in diluted plasma (Method I). In a clincal material, this capacity showed a strong positive correlation (r=0.85) to the thrombin-inactivating capacity of diluted heparinized plasma (heparin cofactor activity) and apparently reflects antithrombin III (At-III) concentration. In the absence of heparin, the inactivation of factor xa occurs slowly. With an incubation of 5 min, about 25% of Xa is inactivated, and this assay reflects initial inactivation of Xa (Method II). With this method, a positive, but less strong correlation to the thrombin-inactivating capacity was found (r=0.58), indicating that inhibitors different from At-III accounts for a minor part of the initial inactivation. Determinations in plasma, in which At-III was removed by immunoadsorption, indicated that At-III accounts for about 80% of the initial inactivation. The results of the assays are not significantly influenced by varying concentrations of fibrinogen, fibrinogen degradation products or heparin in the test plasma.
mesh_terms: D000577:Amides / Q000378:metabolism*; D000990:Antithrombin III / Q000032:analysis; D000991:Antithrombins / Q000172:deficiency; D005170:Factor X / Q000037:antagonists & inhibitors*; D005338:Fibrin Fibrinogen Degradation Products / Q000032:analysis; D005340:Fibrinogen / Q000032:analysis; D006493:Heparin / Q000494:pharmacology; D006801:Humans
chemical_list: D000577:Amides; D000991:Antithrombins; D005338:Fibrin Fibrinogen Degradation Products; D000990:Antithrombin III; D005170:Factor X; D005340:Fibrinogen; D006493:Heparin