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.
Why matched laboratory inputs matter
Patient PT and control PT must use the same time unit. The ratio cancels that unit, but this form labels both as seconds to match common reports. A PT activity percentage, PT ratio, or INR already reported by the laboratory is not a substitute for one of the seconds fields.
Control PT is not a universal value. Laboratories determine a mean normal or control value under their method. ISI characterizes thromboplastin sensitivity relative to an international reference, and calibration can depend on the reagent and instrument system. Combining patient PT from one laboratory with a control or ISI from another can produce a misleading calculation.
The implementation accepts positive finite PT values and an ISI from 0.8 through 3.0 inclusive. It divides patient PT by control PT, raises that ratio to the ISI power, and rounds INR and PT ratio to two decimals. Passing these input checks does not verify specimen quality, calibration, quality control, or clinical plausibility.
INR was developed to improve comparability for vitamin K antagonist monitoring, especially warfarin. It does not provide a universal measure for all anticoagulants or bleeding disorders. Direct oral anticoagulants, heparins, liver disease, vitamin K deficiency, factor deficiencies, inhibitors, disseminated coagulation problems, and other conditions can affect clotting tests in ways this number alone cannot interpret.
Therapeutic targets are patient- and indication-specific. They can differ by condition, valve type, procedure, bleeding risk, medication plan, and clinician instructions. This calculator deliberately does not label a target range or recommend a dose. The laboratory result and anticoagulation service should guide care.
Preanalytical and analytical issues can change PT. Collection tube fill, citrate concentration, hematocrit, sample mixing, delays, storage, contamination, and instrument or reagent performance matter. A home testing device has its own quality checks and instructions. This tool cannot identify an invalid sample or device problem.
A result may be clinically urgent because of bleeding, clotting symptoms, a very abnormal lab value, medication error, injury, or planned procedure. Do not wait for an independent web calculation to decide what to do. Follow the anticoagulation team's instructions and use emergency services for severe bleeding, signs of stroke, breathing trouble, chest pain, collapse, or other urgent symptoms.
Small arithmetic differences can arise because the laboratory uses unrounded control, calibration, and PT values. A lab-reported INR should take priority over a number reconstructed from rounded report components. If results disagree, ask the laboratory or treating team rather than selecting the preferred number.
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.
- Using a generic normal PT instead of the control or mean normal PT tied to the laboratory method.
- Copying an ISI from a package or website that does not match the reagent and instrument combination used for the sample.
- Assuming INR standardizes every cause of prolonged PT or every anticoagulant therapy.
Limitations
This calculator applies (patient PT divided by control PT) raised to ISI, using positive numeric PT values and ISI from 0.8 through 3.0. It cannot verify that patient PT, control PT, reagent, instrument, calibration, and ISI match. It does not assess specimen collection, citrate fill, hematocrit, storage, assay interference, home-meter quality, or laboratory controls. INR standardization has a specific clinical context and does not interpret all anticoagulants, liver disease, factor disorders, bleeding, clotting, pregnancy, pediatrics, procedures, or acute illness. It does not provide a therapeutic range, diagnosis, dose, or management plan.
Embed this calculator on your site
Drop this single line where you want the calculator to appear. It is responsive, mobile-friendly, resizes automatically, and is free to use with attribution.
<script src="https://calctide.com/embed.js" data-tool="inr-calculator" async></script>Preview the embed at /embed/inr-calculator/.