INR — Normal Range & Interpretation

Clinical Context

INR, the international normalized ratio, standardizes prothrombin time across laboratories and reflects the extrinsic and common coagulation pathways. It measures how long plasma takes to clot relative to a control, making it the primary tool for monitoring warfarin therapy and assessing hepatic synthetic function.

Elevated INR occurs with warfarin therapy, vitamin K deficiency, liver disease, disseminated intravascular coagulation, malabsorption, and antibiotic use that disrupts gut flora. Drug interactions with amiodarone, fluconazole, metronidazole, and sulfa agents potentiate warfarin and raise INR. Decreased INR signals subtherapeutic anticoagulation and appears with increased vitamin K intake from leafy greens, enzyme-inducing drugs like rifampin and carbamazepine, and nonadherence.

On the AANP exam, expect therapeutic targets: 2.0 to 3.0 for atrial fibrillation, venous thromboembolism, and bioprosthetic valves, and 2.5 to 3.5 for mechanical mitral valves. Expect questions on managing supratherapeutic INR, including holding warfarin for values above 4.5, administering oral vitamin K for INR above 10 without bleeding, and giving prothrombin complex concentrate with fresh frozen plasma for active hemorrhage. Questions also address dietary counseling on consistent vitamin K intake, recognizing drug interactions, and distinguishing warfarin monitoring from direct oral anticoagulants, which require no routine INR checks.

Related Labs

• PT — seconds
• aPTT — seconds
• D-dimer — μg/mL

Sources

• MedlinePlus
• ARUP Consult