International normalised ratio (INR)

Last updated: Tuesday, 07, August, 2007
Item Process

4.5 mL blood added to 0.5 mL citrate.


The result for the prothrombin time is expressed as a prothrombin ratio (PR - clotting time for patient plasma divided by time for control plasma). A correction factor (International Sensitivity Index or ISI) is applied to the prothrombin ratio, as the sensitivity of commercial thromboplastin reagents is variable, and the result issued as an INR (INR=PRISI).  This allows for better standardisation across laboratories and thromboplastins.

See Prothrombin time.

Therapeutic Interval

The therapeutic interval for oral anticoagulant therapy is as follows:

therapy after DVT or pulmonary embolism 2.0-3.0;
preventing systemic embolism;
atrial fibrillation 2.0-3.0;
valvular heart disease 2.0-3.0;
after myocardial infarction 2.0-3.0;
tissue heart valves (first three months) 2.0-3.0;
bileaflet mechanical heart valves (aortic) 2.5-3.5;
mechanical prosthetic heart valve (high risk) 3.0-4.5; and
thrombosis in antiphospholipid antibody syndrome 2.0-3.0, although a target of 3.5 maybe indicated and is reasonable as long as the bleeding risk at the higher intensity of anticoagulation is considered.


Monitoring oral anticoagulant therapy and part of broader testing of anticoagulation.


The therapeutic intervals represent the levels at which therapy should be effective without an excessive risk of bleeding.

Although expression of the results as an INR 'corrects' for reagent variability, this is not entirely satisfactory and some of the differences in the literature may relate to reagent sensitivity.


Gallus A et al. Med J Aust 2000; 172: 600-605.

Baglin T et al. BJH 2005: 277-285.