Automated antithrombin III assay with a centrifugal analyser

The factor Xa inactivating function of antithrombin III is measured automatically by an amidolytic method, adapted to a centrifugal analyser. Plasma is diluted in buffer with heparin. In stage I, diluted plasma is incubated with excess factor Xa. Heparin accelerates the saturation of antithrombin with factor Xa. In stage II, remaining factor Xa is determined with the chromogenic substrate Bz-Ile-Glu-Gly-Arg-pNA. The precision of the present assay compares favourably with that of the clotting assays and immunoassay. There is a close correlation (r = 0.82) between the results obtained with this assay and the immunoassay of antithrombin III.