Calculate International Normalized Ratio from PT and ISI, assess warfarin therapeutic range, and get dose adjustment guidance by indication.
The International Normalized Ratio (INR) is the standardized measurement of blood coagulation time, derived from the prothrombin time (PT) test. It was developed to ensure consistent anticoagulation monitoring across different laboratories and reagents by normalizing results using the International Sensitivity Index (ISI) of each thromboplastin reagent.
INR monitoring is essential for patients on warfarin (Coumadin) therapy, which remains the most widely prescribed oral anticoagulant for conditions including atrial fibrillation, deep vein thrombosis, pulmonary embolism, and mechanical heart valve replacement. The therapeutic INR range varies by indication — typically 2.0–3.0 for most conditions, and 2.5–3.5 for mechanical heart valves.
This calculator evaluates your INR against indication-specific therapeutic ranges, provides evidence-based warfarin dose adjustment recommendations based on the ACP/ACC/AHA guidelines, estimates bleeding risk at supratherapeutic levels, and offers reversal strategies when INR values exceed safe thresholds. It also includes a comprehensive drug interaction reference, since warfarin has one of the highest interaction potentials of any medication.
This calculator provides instant therapeutic range assessment tailored to your specific anticoagulation indication, evidence-based dose adjustment recommendations, bleeding risk stratification, and reversal strategy guidance. The comprehensive drug interaction reference helps identify common medications that may affect INR stability. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation.
INR = (Patient PT / Control PT)^ISI. Therapeutic range: 2.0–3.0 (most indications) or 2.5–3.5 (mechanical heart valves). Dose adjustments based on American College of Chest Physicians (ACCP) guidelines.
Result: Mildly Supratherapeutic, Moderate bleeding risk, Hold 1 dose and reduce weekly dose by 10–15%, Recheck in 1 week
An INR of 3.8 exceeds the therapeutic range of 2.0–3.0 for atrial fibrillation. The dose should be held for one day, then the weekly total reduced by 10–15%, with a recheck in 1 week.
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A normal INR is 0.8–1.2. Healthy individuals with normal coagulation typically have an INR of approximately 1.0.
A supratherapeutic INR (>3.0 for most indications) increases bleeding risk. At INR >4.5, the risk rises significantly. Above 9.0, there is high risk of major hemorrhage requiring urgent reversal.
Every 4–6 weeks when stable and therapeutic. More frequently (1–2 weeks) after dose changes, illness, new medications, or supratherapeutic values.
Foods rich in vitamin K (leafy greens like kale, spinach, broccoli) decrease INR. The key is maintaining consistent vitamin K intake rather than avoidance.
The International Sensitivity Index (ISI) standardizes PT results across different laboratory reagents. Each thromboplastin reagent has its own ISI; values closer to 1.0 are more sensitive.
Direct oral anticoagulants (DOACs like apixaban, rivarelbaan) have replaced warfarin for many patients with atrial fibrillation and VTE. However, warfarin remains required for mechanical heart valve patients.
For mild elevations (INR 4.5–6.0), holding warfarin 1–2 doses may suffice. For INR >6.0, oral vitamin K (2.5–5 mg) is given. For active major bleeding or INR >9.0, IV vitamin K and 4-factor prothrombin complex concentrate (PCC) are used.