International Normalized Ratio
Enter the patient’s prothrombin time (PT), the laboratory mean normal PT, and the reagent international sensitivity index (ISI) to calculate INR.
Calculated INR
2.00
How to Use This Calculator
Gather laboratory data
Record the patient’s PT, the laboratory’s mean normal PT, and the lot-specific ISI supplied with the thromboplastin reagent.
Enter the values
Input PT and mean PT in seconds and ISI as provided (typical range 0.9–1.7). The calculator outputs INR instantly.
Interpret in context
Compare the resulting INR with the therapeutic target for the patient’s indication (e.g., AFib, mechanical valve) and adjust warfarin dosing accordingly.
Formula
INR = (Patient PT ÷ Mean normal PT)ISI
Use consistent units (seconds) for PT values. ISI varies with reagent lot and instrument calibration.
Full Description
The international normalized ratio standardizes prothrombin time across laboratories by adjusting for thromboplastin sensitivity (ISI). Accurate INR monitoring is essential for patients on vitamin K antagonists. Unexpected INR changes should prompt evaluation of diet, drug interactions, liver function, and adherence. Manage extreme INR values according to evidence-based protocols to minimize bleeding or thrombotic risk.
Frequently Asked Questions
What is the therapeutic INR range?
Most indications target 2.0–3.0. Mechanical mitral valves or antiphospholipid syndrome may require 2.5–3.5. Always confirm with guidelines.
How often should INR be checked?
Stable patients on warfarin are typically checked every 4–12 weeks. Unstable INRs or medication changes warrant more frequent monitoring.
Does liver disease affect INR?
Yes. Hepatic dysfunction reduces clotting factor synthesis, raising INR independent of warfarin. Manage based on comprehensive clinical assessment.
What if the lab doesn’t supply ISI?
Contact the laboratory for reagent-specific ISI. Using a default value may produce inaccurate INR results.