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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

  • The 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 .