The Prothrombin Time (PT) is a measure of the extrinsic coagulation pathway. It is routinely used to monitor therapy with coumarins/warfarin for anticoagulation.

Whereas the PT is the actual time in seconds, for an anticoagulated sample of plasma or whole blood to clot after it is added to a thromboplastin reagent, the International Normalized Ratio (INR) is calculated based upon a formula that relates to the International Sensitivity Index (ISI) of the specific thromboplastin used by the laboratory to the PT of the patient compared to a normal PT in seconds.

Notes on ISI:

The manufacturer of the thromboplastin reagent determines the ISI by comparing each batch of reagent to WHO reference plasma and then assigning an ISI value to that lot of reagent.TheCollege of American Pathologists (CAP) Conference on Laboratory Monitoring of Anticoagulant Therapy held in 1998 recommended that ISI values should usually be in the range of 0.9 to 1.7.

The INR is calculated from the following formula:

INR=(PT patient/PT normal) ^ISI

• PT patient = patient's measure PT (seconds)
  
• PT normal = laboratory's geometric mean value for normal patients (seconds)
  
• ISI = International Sensitivity Index

References:

Nichols W. L, Bowie E. J. W. Standardization of the prothrombin time for monitoring orally administered anticoagulant therapy with use of the international normalized ration system. Mayo Clinical Procedure.1993;68:897-98.
Ortel, L.B. International normalized ratio (INR); An improved way to monitor oral anticoagulant therapy. Nurse Practitioner.
1995;20(9):15-22.