The INR component of the laboratory PT/INR result is a calculated value that is used by the clinician to monitor warfarin therapy and adjust dosage as dictated by clinical status. An INR of 2.0 - 3.0 is often desired as the therapeutic range. The following formula is used by the clinical laboratory to derive an INR value. The INR must be adjusted for every new lot of PT reagent (thromboplastin).
INR= (PT of patient/PT of geometric mean of the normal population)ISI
The International Sensitivity Index (ISI) value, is provided by the reagent manufacturer as the relative sensitivity of the reagent itself. When opening a new lot of PT reagent (thromboplastin), it is essential to verify that the ISI value provided by the reagent manufacturer is being used with that reagent lot to prevent the reporting of erroneous INR results, which may have serious consequences for the patient.
The INR is used to standardize PT results, and in turn, anticoagulant therapy, across laboratory instrumentation, methodologies, and locale.