Oral anticoagulant control with a chromogenic substrate: calibration, precision and cost

Prothrombin time ratios, and bioassays and chromogenic (S2222) assays for factors VII and X, were obtained on 23 newly anticoagulated patients and 23 patients on long term oral anticoagulants. Factor levels were plotted against prothrombin time ratios. For factor VII, a different relationship was apparent for newly anticoagulated and for stabilized patients by both assay methods, but for factor X the same relationship was found. Chromogenic factor-X levels were calibrated against prothrombin time, and a prescribing exercise showed reasonable agreement between doses based on prothrombin time ratios and on assays for both clinical groups; factor-VII levels could not be used because of the different relationship between newly anti-coagulated and long-term patients. Despite the high precision of reading the chromogenic end point, chromogenic assays were not more precise than bioassays. With S2222, the cost of automated anticoagulant control would be more expensive than control by manual prothrombin times.