International Sensitivity Index
Quick values measured with different thromboplastins cannot be directly compared with one another, as for instance sensitivity to clotting factors may differ. To render coagulation times as comparable as possible, the World Health Organization (WHO) approved in 1983 a standard reference thromboplastin. Every manufacturer of thromboplastin must calibrate his reagent against the WHO standard (2 references exist: one for human recombinant based thromboplastins, one for rabbit brain based ones). The value obtained is known as the International Sensitivity Index (ISI). This enables the various sensitivities of the thromboplastins to be ascertained and it will be used to calculate the INR.
WHO recommends ISI to be < 1.7. In many countries, a low (1.0) ISI is favored.
The INR is a standardization method that attempts to minimize differences between thromboplastin reagents through a calibration process in which all commercial thromboplastins are compared with an International Reference Preparation (IRP) maintained by the World Health Organization (WHO).
The INR method is not perfect in correcting for differences between different laboratories utilizing different thromboplastin reagents, but it does reduce the variation between different laboratories and provides clinically useful results. To minimize this variation, each monitoring site should use results from only one type of instrument and thromboblastin reagent origin for each patient.