INR

INR = (patient PT / control PT)^ISI, where ISI (International Sensitivity Index) corrects for variability between different thromboplastin reagents used in PT testing across labs, allowing standardized comparison of results worldwide. The PT/INR measures the extrinsic and common coagulation pathways, reflecting the activity of factors I (fibrinogen), II (prothrombin), V, VII, and X. Clinically, INR is most commonly used to monitor warfarin (vitamin K antagonist) therapy—therapeutic range is typically 2.0-3.0 for most indications (e.g., atrial fibrillation, DVT/PE treatment) and 2.5-3.5 for mechanical heart valves. An elevated INR indicates increased bleeding risk and can result from warfarin overdose, vitamin K deficiency, liver disease (decreased synthesis of clotting factors), or DIC. A normal INR is approximately 0.8-1.2 (near 1.0 in healthy individuals not on anticoagulants). INR is also used as part of the MELD score to assess severity of liver disease and prognosis in cirrhosis. Reversal of elevated INR due to warfarin can be achieved with vitamin K, fresh frozen plasma (FFP), or prothrombin complex concentrate (PCC) depending on urgency and bleeding severity. Unlike PT alone, INR allows for consistent monitoring across different labs and reagents, making it the preferred metric for long-term anticoagulation management. It's important to note that INR does NOT reliably reflect the anticoagulant effect of direct oral anticoagulants (DOACs) like dabigatran, rivaroxaban, or apixaban, which work through different mechanisms (direct thrombin or factor Xa inhibition).