A comparison of five commercial thromboplastins: ISI re-evaluation on an automated coagulometer.

Five commercial rabbit brain thromboplastins were compared with an International Reference Preparation on an ACL coagulometer, using 90 patients stabilized on warfarin and 22 normal individuals. The prothrombin times were converted to INRs using the thromboplastin manufacturers' quoted ISI. The quoted ISIs were reassigned using orthogonal regression analysis and then used to recalculate INRs for patient and commercial INR control plasmas. This showed that the manufacturers' quoted ISIs and the INR control plasma results were inconsistent. With one thromboplastin the manufacturers quoted ISI changed from 1.17 to 1.05 whilst the control plasma results changed from an INR of 4.3 to an INR of 3.7 (manufacturer's INR, 3.3). In most routine laboratories ISI reassignment is not practical. We conclude that the availability of a reliable plasma calibrant is essential for the accurate calculation of INRs at a local level.