INR Calculator
A patient PT of 24 seconds with a control PT of 12 seconds and a reagent ISI of 1.0 gives an estimated INR of 2.0. This INR calculator uses the WHO INR formula to estimate the International Normalized Ratio from a patient prothrombin time, a normal control PT, and the reagent's International Sensitivity Index (ISI). Enter all three values, and treat the result as an educational estimate rather than a lab-reported INR.
Quick answer
The WHO INR formula estimates INR as the patient PT divided by the control PT, raised to the power of the ISI.
What this tells you
- •The WHO INR formula estimates INR as the patient PT divided by the control PT, raised to the power of the ISI.
- •This tool expects patient PT and control PT in seconds and the ISI as the reagent's sensitivity value.
- •The ISI is commonly used only within the reagent's typical range of about 0.8 to 3.0.
- •The result is an educational estimate, not a substitute for a lab-reported INR.
How to Use
- 1Enter the patient's prothrombin time (PT) in seconds.
- 2Enter the laboratory's normal or control PT in seconds.
- 3Enter the reagent's International Sensitivity Index (ISI), found on the reagent packaging or lab report.
- 4Calculate to see the estimated INR and the underlying PT ratio.
- 5If the ISI falls outside the common reagent range, or the result does not match your lab report, rely on the lab-reported INR instead of this estimate.
How It Works
Formula
WHO INR formula: INR = (patient PT / control PT) ^ ISI
Patient PT and control PT in seconds, ISI from the reagent
Example: (24 / 12) ^ 1.0 = 2.0The formula first divides the patient's PT by the laboratory's normal control PT to get the PT ratio. It then raises that ratio to the power of the reagent's ISI, which corrects for differences in sensitivity between thromboplastin reagents used at different labs. This standardization is why INR values are comparable across laboratories even when they use different PT reagents, but the calculation still depends on accurate PT and ISI inputs.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
ISI of 1.0 example
The PT ratio is 24 / 12 = 2.0. Raised to the power of 1.0, the INR stays 2.0.
Higher ISI example
The PT ratio is 28 / 12 = 2.33. Raised to the power of 1.2, the estimated INR is 2.76.
Common INR Calculation Checks
Use these quick checks before relying on a WHO INR formula estimate from a PT and ISI.
| Check | Why it matters | How this tool handles it |
|---|---|---|
| Patient PT and control PT in seconds | The PT ratio step assumes both times use the same unit. | Enter patient PT and control PT in seconds only. |
| Positive PT values | A PT of zero or less is not a valid clotting time. | The tool rejects PT values at or below 0. |
| ISI in the common reagent range | Most modern thromboplastin reagents report an ISI between about 0.8 and 3.0. | The tool rejects an ISI outside 0.8 to 3.0. |
| Result matches a plausible INR | A non-finite or non-positive result usually means the inputs do not fit the formula. | The tool rejects a computed INR that is not a positive finite number. |
A valid formula estimate can still differ from a lab-reported INR because of reagent calibration, instrument method, and quality control differences.
Common mistakes
- Mixing PT values measured in different units, such as seconds and a percentage of activity.
- Using an ISI from a different reagent than the one used for the patient PT and control PT.
- Treating an estimated INR as a dosing instruction instead of an educational estimate.
Limitations
This calculator applies the WHO INR formula to the patient PT, control PT, and ISI you enter. It does not verify that the PT values came from the same reagent as the ISI, and it does not estimate INR when the ISI falls outside the common reagent range of 0.8 to 3.0 or when the PT values are not positive. The estimate assumes accurate, matched inputs, and it can differ from a lab-reported INR because of reagent calibration, instrument method, specimen handling, and other clinical factors.