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INR Calculator

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Calculates the International Normalized Ratio (INR) for patients on warfarin anticoagulant therapy. This calculator uses the prothrombin time (PT) of the patient and the International Sensitivity Index (ISI) of the thromboplastin reagent used in the PT test.

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How To Use

How to use this calculator

Start with your values, or try a preset.

Step 1

Enter Patient's Prothrombin Time (PT) in Seconds, Control Prothrombin Time (PT) in Seconds, and International Sensitivity Index (ISI) of the Reagent in the units shown.

Step 2

Read International Normalized Ratio (INR) in the results panel. Results update as you change the inputs.

Step 3

Choose a preset to load an example, then adjust the values for your situation.

Worked Examples

Example calculations

See the inputs and their calculated results.

Example 1

Patient's Prothrombin Time (PT) in Seconds: 15; Control Prothrombin Time (PT) in Seconds: 12; International Sensitivity Index (ISI) of the Reagent: 1

International Normalized Ratio (INR): 1.25

Example 2

Patient's Prothrombin Time (PT) in Seconds: 20; Control Prothrombin Time (PT) in Seconds: 12; International Sensitivity Index (ISI) of the Reagent: 1.2

International Normalized Ratio (INR): 1.845943905

Example 3

Patient's Prothrombin Time (PT) in Seconds: 25; Control Prothrombin Time (PT) in Seconds: 12; International Sensitivity Index (ISI) of the Reagent: 1.5

International Normalized Ratio (INR): 3.007032652

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Important

Use the result as an estimate and confirm any medical decisions with a qualified professional.