INR

Keywords: International normalised ratio

Specimen:

4.5 mL blood added to 0.5 mL citrate.

Method:

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. 

Application:

Monitoring warfarin therapy and part of broader testing of anticoagulation.

Interpretation:

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. 

Reference:

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