PT to INR Calculator

About PT, INR, and ISI

Prothrombin time (PT) measures how long plasma takes to clot after adding thromboplastin reagent, reflecting the extrinsic and common coagulation pathways. Because different thromboplastin reagents vary in sensitivity, a raw PT in seconds isn't comparable between labs. The International Sensitivity Index (ISI) calibrates each reagent against a WHO reference thromboplastin, and the INR formula uses it to produce a standardized ratio that means the same thing regardless of which lab or reagent was used — this is what makes INR reliable for monitoring warfarin therapy across different laboratories.

Causes of a prolonged PT/INR

• Vitamin K antagonist therapy (warfarin) — the primary indication for INR monitoring
• Vitamin K deficiency — malnutrition, malabsorption, prolonged antibiotic use, cholestasis
• Liver disease — reduced synthesis of vitamin K-dependent clotting factors (II, VII, IX, X)
• Disseminated intravascular coagulation (DIC) and consumptive coagulopathies
• Specific factor deficiencies (especially factor VII) or factor inhibitors
• Supratherapeutic anticoagulation, drug interactions with warfarin

Limitations

• INR standardization was developed and validated specifically for monitoring vitamin K antagonists (warfarin) — it is not designed to standardize PT for other causes of coagulopathy (e.g. liver disease), though it is still commonly reported and used clinically in that context (e.g. MELD score)
• INR does not reliably reflect anticoagulant effect for direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, or dabigatran — different monitoring approaches are needed for these agents
• Point-of-care INR devices may differ slightly from central laboratory values
• An unexpectedly high or low result should prompt clinical correlation and consideration of repeat testing