Keywords: International normalised ratio


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 varies according to the clinical indication. 


Monitoring warfarin therapy and part of broader testing of anticoagulation.


Although expression of the results as an INR 'corrects' for reagent variability, some difference in INR results can be observed,  possibly due to reagent sensitivity. 


Antithrombotic therapy for VTE disease. Antithrombotic therapy and Prevention of Thrombosis 9th ed: American college of chest physicians evidence-based clinical practice guidelines. 2012; 141. 

Keeling D et al. Guidelines for the investigation and management of antiphospholipid syndrome. BJH. 2012; 157: 47-58

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