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

How to Subscribe
MLS & MLT Comprehensive CE Package
Includes 154 CE courses, most popular
$95Add to cart
Pick Your Courses
Up to 8 CE hours
$50Add to cart
Individual course$20Add to cart
Need multiple seats for your university or lab? Get a quote
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.

Learn more about Fundamentals of Hemostasis (online CE course)
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 .