How can comparability be brought about? What does ISI mean?
By using the International Sensitivity Index (ISI), the different INR can be compared. The results are then standardised. Each manufacturer matches its reagent to a standard reagent, the reference thromboplastin, proposed by the WHO. The ISI of one (the human origin one) of the reference thromboplastin is very close to 1. Each manufacturer determines the ISI value of its reagent lot by matching its thromboplastin production lot to a master lot which is measured against the WHO reference thromboplastin. The ISI value determined this way enters the calculation of INR as an exponent value depending on lot and method.
