We performed an extensive study to check international normalized ratio (INR) recovery using three different rabbit thromboplastin reagents of different sensitivities [international sensitivity index (ISI) circa 1.1, 1.3 and 2.0] on two different instrument types. Both instrument types used an optical end point: one nephelometric at 660 nm, one photometric at 880 nm. Each thromboplastin reagent in the study had an instrument-specific ISI value assigned to it by the reagent manufacturer. Samples obtained from stable orally anticoagulated patients were tested on four different instruments of each type with the three different sensitivity reagents. Excellent correlation between INR values was seen between all results (r values between 0.97 and 0.99). Regression analysis gave slopes between 0.92 and 1.07, and a maximum deviation of intercept from zero of 0.21. Variations in CV of less than 2% were seen within each instrument type. This study shows that if instrument-specific ISI values are used, comparable INR values can be obtained using reagents of different sensitivity on multiple instruments of the same or different types. This study demonstrates the validity of the INR/ISI system.