Direct international normalized ratio determination using multicalibrators is more responsive than the conventional method for measuring prothrombin time

Direct international normalized ratio (INR) determination using certified INR plasmas was shown to improve precision and accuracy. We evaluated the utility of a multicalibrator in determining INR. INR values were measured in 493 blood samples from patients subjected to anticoagulation therapy (320) and control subjects (173). Study was performed using CA-7000 coagulation analyzer (Sysmex, Japan) with Thromborel S (Dade Behring, Germany). Direct INR values were obtained using PT-Multi Calibrator (Dade Behring) composed of five lyophilized calibrant plasmas. Conventional INR values were calculated from mean normal prothrombin time and instrument/reagent-specific international sensitivity index (ISI). We compared the difference between the INR results obtained with the two methods. The mean INR value of direct INR method was significantly higher than that of conventional method. The differences in values (direct INR – conventional INR) generated using the two methods increased in proportion to the INR values. Elevation of INR was observed in data obtained with the direct INR method, compared with conventional INR values. Accordingly, we conclude that direct INR method is more responsive than conventional method in determining INR.