The laboratory result's International Normalized Ratio (INR) component is a calculated value the clinician uses to monitor anticoagulant therapy and adjust dosage as clinical status dictates. An INR of 2.0–3.0 is often desired as the therapeutic range.
The clinical laboratory derives an INR value using the following formula. The INR must be adjusted for every new lot of PT reagent.
INR= (PT of patient/PT of the geometric mean of the average 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.
