PT Ratio = Patient PT ÷ Control PT
INR = (PT Ratio)ISI
Prothrombin Index (%) = (Control PT ÷ Patient PT) × 100
ISI is method-dependent and supplied by the reagent manufacturer or laboratory validation.
- Enter the patient’s PT result in seconds.
- Enter the control PT (or MNPT) used by your lab.
- Enter the ISI value for the reagent/instrument pair.
- Select rounding and an optional context note.
- Click Calculate to show results above the form.
- Use Download CSV or Download PDF for reporting.
| Patient PT (s) | Control PT (s) | ISI | PT Ratio | INR | Prothrombin Index (%) |
|---|---|---|---|---|---|
| 18.5 | 12.0 | 1.1 | 1.542 | 1.610 | 64.9 |
What Prothrombin Time Measures
Prothrombin time (PT) reports how long citrated plasma takes to clot after tissue factor and calcium are added. It reflects factor VII and the common pathway (X, V, II, fibrinogen). Many laboratories report PT in seconds plus a ratio, because seconds alone vary with reagents and instruments. Typical adult reference PT is roughly 11-13.5 seconds, but each lab sets its own interval. PT is part of screening panels alongside aPTT and fibrinogen for bleeding evaluation as needed.
Why INR Standardization Matters
The international normalized ratio (INR) reduces between-lab differences by scaling the PT ratio with the reagent's International Sensitivity Index (ISI). INR is dimensionless and is commonly used when monitoring vitamin K antagonist therapy. For people not receiving anticoagulants, INR is often near 0.8–1.2, yet "normal" still depends on the local reference interval and method performance. Common therapeutic targets vary by indication and must be set by the treating team.
How ISI and Control PT Affect Output
This calculator applies: PT Ratio = patient PT ÷ control PT, then INR = (PT Ratio)ISI. A higher ISI means the reagent is less sensitive, so the same PT ratio yields a different INR. Example: ratio 1.50 gives INR 1.50 at ISI 1.0, but about 1.66 at ISI 1.2. Accurate control PT (or MNPT) is therefore as important as the patient value. Small timing errors can matter.
Using the Log for Trend Review
Consistent documentation helps interpret repeat testing. The built-in log stores timestamped inputs, computed ratio, INR, and prothrombin index. Exporting CSV supports audits, quality meetings, or charting outside the page. If a patient's PT seconds drift upward while control PT is stable, the ratio and INR rise together, making trends easier to spot than raw seconds alone.
Flags That Merit Rechecking
Results deserve review when pre-analytical or analytical factors could bias clotting times. Common issues include underfilled citrate tubes, delayed centrifugation, high hematocrit (altering plasma:citrate), heparin contamination, liver dysfunction, vitamin K deficiency, or reagent lot changes. If values conflict with the clinical picture, repeat sampling and confirm the correct ISI and control PT before interpretation.
What does the PT ratio represent?
It is the patient PT divided by the control PT (or MNPT). The ratio normalizes seconds for the local method, and it is the base value used to compute INR with the ISI exponent.
Where do I get the ISI value?
ISI is supplied by the thromboplastin manufacturer and may be specific to an instrument. Many labs verify it during validation. Use the value printed on the reagent insert or confirmed in your laboratory documentation.
Why can two laboratories report different INR values?
Different reagents, instruments, ISI values, and control PT/MNPT calculations change the PT ratio and exponent step. Pre-analytical factors, such as tube fill and processing time, can also shift clotting times.
Should I enter control PT or MNPT?
Either works if it matches how your lab defines the denominator for PT ratio. MNPT is often used for INR standardization. Use the same reference consistently across results to avoid artificial trend changes.
How should I choose rounding?
For reports, many teams round INR to two or three decimals and PT to one decimal, but policies vary. Choose the rounding that matches your laboratory information system and audit requirements.
Can this tool replace clinical interpretation?
No. It provides calculations only and cannot account for diagnosis, bleeding risk, medications, or comorbidities. Always review results with a qualified clinician and follow your local laboratory and treatment protocols.