Notes:
The use of the International Normalized Ratio (INR) in the monitoring of oral anticoagulant therapy allows for a more accurate comparison between laboratories, and with recommended INR ranges in the literature. The Prothrombin Time (PT) ratio can vary between laboratories, and with time in the same laboratory as a different batch of thromboplastin reagent is used.

The formula and the nomogram show how the INR and PT ratio are related. The INR can be calculated when the ISI of the laboratory’s thromboplastin reagent is known.33 An analogy can be made when purchasing a pair of shoes in a foreign country. You will pay in a foreign currency (PT), which will vary depending on point in time and country. If you know the exchange rate (ISI), you can always convert back to a price in dollars (INR).

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Acknowledgments:
The content of this presentation was prepared by Dr. Gregory Albers of the Stanford Stroke Center.