International Normalized Ratio Calculator
Calculate INR from patient PT and mean normal PT for anticoagulation monitoring. Enter values for instant results with step-by-step formulas.
Reviewed by Rahul Singh, Health & Wellness Specialist
Formula
INR = (Patient PT / Mean Normal PT) ^ ISI
Where Patient PT is the measured prothrombin time in seconds, Mean Normal PT is the geometric mean of normal PT values for the laboratory, and ISI (International Sensitivity Index) is specific to the thromboplastin reagent used. The ISI corrects for reagent variability, standardizing results across laboratories.
Worked Examples
Example 1: Standard INR Calculation for DVT Treatment
Problem:A patient on warfarin for DVT has a PT of 24 seconds. The laboratory mean normal PT is 12 seconds and the thromboplastin ISI is 1.0. Calculate the INR and assess therapeutic status.
Solution:PT Ratio = Patient PT / Mean Normal PT = 24 / 12 = 2.00\nINR = (PT Ratio)^ISI = (2.00)^1.0 = 2.00\nTarget range for DVT: 2.0-3.0\nStatus: Therapeutic (at lower end of range)\nAssessment: INR is at the lower threshold of the therapeutic range
Result:INR = 2.00 - Therapeutic for DVT treatment (target 2.0-3.0)
Example 2: Elevated INR with Non-Unit ISI
Problem:A patient on warfarin for mechanical aortic valve has a PT of 20 seconds. Laboratory mean normal PT is 11.5 seconds with thromboplastin ISI of 1.2. Determine if the INR is within the target range of 2.5-3.5.
Solution:PT Ratio = 20 / 11.5 = 1.739\nINR = (1.739)^1.2 = 1.739^1.2\nUsing logarithms: ln(1.739) = 0.553, multiply by 1.2 = 0.664\nINR = e^0.664 = 1.94\nTarget range for mechanical valve: 2.5-3.5\nStatus: Subtherapeutic (below 2.5)
Result:INR = 1.94 - Subtherapeutic for mechanical valve (target 2.5-3.5); warfarin dose increase needed
Frequently Asked Questions
What is the International Normalized Ratio (INR)?
The International Normalized Ratio (INR) is a standardized measurement of blood clotting time derived from the prothrombin time (PT) test. It was introduced by the World Health Organization in 1983 to allow comparison of coagulation results between different laboratories and different thromboplastin reagents. The INR is calculated by raising the PT ratio (patient PT divided by mean normal PT) to the power of the International Sensitivity Index (ISI) of the thromboplastin reagent used. A normal INR for a person not on anticoagulation therapy is approximately 0.8-1.2. The INR is the primary laboratory test used to monitor patients on warfarin (Coumadin) and other vitamin K antagonist anticoagulants.
References
- Hirsh J et al. American Heart Association/American College of Cardiology Foundation guide to warfarin therapy. Circulation. 2003;107(12):1692-1711
- WHO Expert Committee on Biological Standardization. Guidelines for thromboplastins and plasma used to control oral anticoagulant therapy. WHO Technical Report Series 889. 1999
- Holbrook A et al. Evidence-based management of anticoagulant therapy. Chest. 2012;141(2 Suppl):e152S-e184S
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy