INR Calculator
Calculate International Normalized Ratio from patient PT, control PT, and ISI with therapeutic range interpretation.
What Is INR?
The International Normalized Ratio (INR) standardizes prothrombin time (PT) results across laboratories so clinicians can compare clotting values reliably. INR reflects activity in the extrinsic and common coagulation pathways and is essential for monitoring warfarin and assessing bleeding or clotting risk. Related assessments include the DIC Syndrome Calculator.
How INR Is Calculated
INR equals the patient PT divided by the laboratory control PT, raised to the power of the International Sensitivity Index (ISI). The ISI depends on the reagent batch and is usually between 0.9 and 1.7. Enter all three values from your lab report to compute INR instantly.
Interpreting INR Results
An INR near 1.0 is typical for patients not on anticoagulation. Values between 2.0 and 3.0 are commonly targeted for atrial fibrillation, deep vein thrombosis, and pulmonary embolism on warfarin. Higher values increase bleeding risk, while very low values may suggest clotting tendency or liver dysfunction depending on clinical context.
Frequently Asked Questions
What is a normal INR?
For patients not taking anticoagulants, INR is typically around 1.0, often reported as 0.8 to 1.2 depending on the laboratory reference range.
What INR is therapeutic on warfarin?
Most conditions target INR between 2.0 and 3.0. Mechanical heart valves may require higher targets, often 2.5 to 3.5, as directed by a cardiologist.
What does ISI mean in PT testing?
The International Sensitivity Index describes how a particular thromboplastin reagent responds compared with a reference standard. It is required to convert PT to INR.
Can I use this for patients on direct oral anticoagulants?
DOACs such as apixaban and rivaroxaban are not monitored with INR. This calculator applies to warfarin and standard PT-based testing.
What INR level is dangerous?
INR above 4.0 carries significant bleeding risk, and values above 5.0 may require urgent evaluation. Always interpret INR in context with symptoms and recent medication changes.
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