INR Calculator
Calculate the International Normalized Ratio (INR) for monitoring warfarin therapy. Therapeutic INR is 2.0-3.0 for most indications, 2.5-3.5 for mechanical heart valves.
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⚠️For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
INR (International Normalized Ratio) standardizes blood clotting time across laboratories. It is the primary metric for monitoring warfarin therapy. Therapeutic INR is 2.0-3.0 for atrial fibrillation and DVT/PE; 2.5-3.5 for mechanical heart valves. Formula: INR = (Patient PT / Normal PT)^ISI.
Sources: WHO, AHA, ACCP guidelines.
Key Takeaways
- • Therapeutic INR 2.0-3.0 balances clot prevention and bleeding risk
- • INR >5.0 requires dose hold; INR >9.0 may need vitamin K
- • Antibiotics and diet affect INR; maintain consistent vitamin K intake
- • Never adjust warfarin without medical supervision
Did You Know?
How Does INR Work?
PT Ratio
Patient PT divided by lab reference normal PT. Raw PT varies by reagent.
ISI Correction
ISI (International Sensitivity Index) standardizes different thromboplastin reagents. Raising PT ratio to ISI power yields comparable INR.
Target Ranges
Indication-specific: A-Fib 2.0-3.0; mechanical mitral 2.5-3.5. Below target increases thrombosis; above increases bleeding.
Expert Tips
INR Interpretation Table
| INR Range | Status | Action |
|---|---|---|
| <1.5 | Subtherapeutic | Consider dose increase |
| 2.0-3.0 | Therapeutic (A-Fib) | Continue dose |
| 3.5-5.0 | Elevated | Hold 1-2 doses |
| >5.0 | High | Hold, consider vitamin K |
| >9.0 | Critical | Urgent reversal |
Frequently Asked Questions
What is INR?
INR (International Normalized Ratio) standardizes prothrombin time across labs. Therapeutic INR is 2.0-3.0 for most indications; 2.5-3.5 for mechanical heart valves. Normal is 0.9-1.1.
How is INR calculated?
INR = (Patient PT / Normal PT)^ISI. The ISI (International Sensitivity Index) corrects for reagent variability. Example: PT 24 sec, Normal 12 sec, ISI 1.0 → INR = 2.0.
What INR range is therapeutic for warfarin?
For atrial fibrillation and DVT/PE: 2.0-3.0. For mechanical mitral valves and antiphospholipid syndrome: 2.5-3.5. Below 2.0 increases clot risk; above 3.5 increases bleeding risk.
When should INR be checked urgently?
INR >5.0: hold warfarin, consider vitamin K. INR >9.0: urgent reversal. INR <1.5 with high thrombosis risk: consider bridging. Active bleeding: immediate reversal.
What affects INR besides warfarin dose?
Antibiotics, NSAIDs, vitamin K intake, alcohol, liver/kidney disease, and drug interactions. Maintain consistent vitamin K diet and report new medications to your provider.
Can this calculator replace medical advice?
No. This tool is for educational purposes only. Warfarin dosing and INR interpretation require a qualified healthcare provider. Never adjust anticoagulation without medical supervision.
Key Statistics
Official Data Sources
⚠️ Disclaimer: This calculator is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Warfarin dosing and INR interpretation require a qualified healthcare provider. Never adjust anticoagulation without medical supervision.