An Introduction to the Fundamentals of Coagulation: Laboratory Tests of Hemostatic Function

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 .


Review for ASCP Boards or test your clinical laboratory science knowledge with the FREE LabCE.com Quiz Game.

Fundamentals of Hemostasis course details »

Learn more about medical technologist continuing education for MTs, MLTs, and other lab personnel »

Get information on laboratory compliance training for clinical and medical laboratories »