The effect of instruments for prothrombin time testing on the international normalized ratio

Prothrombin time (PT) testing is used for monitoring oral anticoagulant therapy, its result being usually expressed as international normalized ratio (INR). This is done using the international sensitivity index (ISI) specific of thromboplastin employed to carry out the test. In this way a good PT standardization may be achieved although the instruments used to calibrate the thromboplastins might influence the ISI value. Presented at the ‘2nd International Symposium on Standardization and Quality Control of Coagulation Tests: Implications for the Clinical Laboratory’, Rome, September 28–29, 1989.