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The page below is a sample from the LabCE course Fundamentals of Hemostasis. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Basic Laboratory Tests of Hemostatic Function – Prothrombin Time (PT)

The International Normalized Ratio (INR) component of the laboratory result is a calculated value that is 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 .