The International Normalized Ratio (INR) component of the laboratory result is a calculated value used by the clinician to monitor anticoagulant 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.
INR= (PT of patient/PT of geometric mean of the normal population)ISI
- The International Sensitivity Index, or ISI value, is provided by the reagent manufacturer as the relative sensitivity of the reagent itself.
The INR is used to standardize PT results and, in turn, anticoagulant therapy across laboratory instrumentation, methodologies, and locale. Be sure to frequently check that ISI values match those of the lot currently in use, as erroneous results may otherwise occur.
