Abstract
International Normalized Ratio of Prothrombin time (INR) is widely used for the appropriate oral anticoagulant therapy with warfarin in patients with atrial fibrillation. The INR values are affected by the International Sensitivity Index (ISI) since the INR is calculated by (patient PT/normal PT) ISI. Although the thromboplastin reagent with an ISI value of nearly 1.0 is recommended, a comprehensive survey of ISI values in Japan has not been achieved. Therefore, we used a questionnaire survey to investigate thromboplastin reagents and their ISI values at the registered institutions of the J-RHYTHM Registry.
The results were obtained from 152 (96.2%) of 158 institutions. The overall average ISI value was 1.20±0.28 (0.82-1.82) . There were significant differences in the average ISI value among ten geographical divisions by ANOVA (p=0.038) , and between South Kanto with a minimum value of 1.09±0.25 and Hokuetsu with a maximum value of 1.47±0.31 using the post hoc test (p=0.012) . The reagent“Thromborel S”with an ISI value around 1.0 was found in 35% of all, whereas two kinds of high-ISI reagents“Thrombochek PT”and“Thromboplastin C Plus”with ISI values of over 1.5 were employed in total up to 23%. In Hokuetsu and Shikoku, the ratio of these high-ISI reagents were more than 50%, where indicated highest average ISI values and lowest average daily warfarin dose at the baseline of the J-RHYTHM Registry.
The present study indicated that the usage of high-ISI reagents might lead to under-doses of warfarin.
