INR
What it measures, and what changes the number.
The INR (International Normalized Ratio) is a standardized way of reporting the prothrombin time, which measures how long plasma takes to form a clot through the extrinsic and common clotting pathways. Different reagents give different raw clotting times, so the INR adjusts the result using a sensitivity index assigned to each reagent, making results more comparable between laboratories.
It is reported with the prothrombin time and prothrombin activity, and often alongside the APTT and APTT ratio to cover the other clotting pathway. It is widely used to monitor treatment with vitamin K antagonist anticoagulants.
The sample is drawn into a citrate tube that must be filled to the correct level, and it should be processed promptly. Point of care finger prick devices may differ slightly from laboratory results. Reports may print it as INR, International Normalized Ratio or INR (Ratio Internacional Normalizado T.protrombina).
Often read alongside: Prothrombin time, Prothrombin activity, APTT, APTT ratio, D-dimer