Prothrombin Time to Quick % Converter
Prothrombin Time to Quick % Converter
A German report of “Quick 70%” and a British report of “INR 1.2” describe the same patient. This page maps a prothrombin time in seconds onto the Quick value in per cent, using your own laboratory’s calibration anchors — and is honest that the mapping is an approximation, because the Quick curve is not a formula.
Prothrombin time to Quick %
PT seconds → Quick %PT 15.0 s, normal plasma 12.5 s, 1:1 dilution 17.5 s
Formula
PT ratio = PT ÷ MNPT
INR = (PT ratio) ^ ISI
- PT
- the patient’s prothrombin time in seconds
- MNPT
- the clotting time of undiluted normal plasma — the 100% point on the Quick calibration curve, and the same mean normal prothrombin time the INR uses
- PT₅₀
- the clotting time of normal plasma diluted 1:1 — the 50% point on the curve. Taking it from your own laboratory is what makes the answer meaningful, because the curve belongs to the reagent
- the hyperbola
- the Quick curve is built by diluting normal plasma to 100, 50, 25, 12.5 and 6.25% and plotting clotting time against activity. That curve is hyperbolic, not linear, which is why the calculation above fits PT = a + b ÷ activity through two anchors rather than interpolating a straight line
- ISI
- the International Sensitivity Index of the thromboplastin, used for the INR relationship, which is exact. The INR is calculated on the dedicated page rather than repeated here
Worked example
PT 15.0 s, normal plasma 12.5 s, 1:1 dilution 17.5 s
Denominator = 15.0 − (2 × 12.5) + 17.5 = 7.5
Numerator = 100 × (17.5 − 12.5) = 500
Quick ≈ 500 ÷ 7.5 = 67%
The same sample as a ratio: 15.0 ÷ 12.5 = 1.20
And as an INR on a reagent with an ISI of 1.05: 1.20^1.05 = 1.21
So Quick 67% and INR 1.21 are one patient — which is the entire point of the page
How the same sample reads in each convention
| PT (seconds) | PT ratio | INR at ISI 1.05 | Quick % on this model |
|---|---|---|---|
| 12.5 | 1.00 | 1.00 | 100% |
| 15.0 | 1.20 | 1.21 | 67% |
| 17.5 | 1.40 | 1.43 | 50% |
| 25.0 | 2.00 | 2.07 | 29% |
| 30.0 | 2.40 | 2.51 | 22% |
| 37.5 | 3.00 | 3.17 | 17% |
What each number is actually for
| Report | Where you will see it | What it is good for | Where it fails |
|---|---|---|---|
| PT in seconds | Everywhere, usually alongside one of the others | Comparing a patient with themselves on one analyser | Meaningless between laboratories — different thromboplastins give different seconds for identical plasma |
| PT ratio | United Kingdom, Ireland, parts of the Commonwealth | A first sense of how prolonged a screen is | Still reagent-dependent; a ratio of 1.5 on a responsive reagent is not a ratio of 1.5 on an insensitive one |
| INR | International, and the only valid report for vitamin K antagonist monitoring | Comparing anticoagulant intensity between laboratories and over time — the one thing it was calibrated to do | Not validated outside vitamin K antagonist monitoring: in liver disease or on a direct oral anticoagulant the number is reported but not calibrated |
| Quick % | Germany, Austria, Switzerland, France, much of central Europe | A familiar scale for clinicians trained on it, where a normal is above 70% | Read off a reagent-specific dilution curve, so it is not comparable between laboratories and cannot be converted exactly to anything |
Why there is no exact conversion, and what this page does instead
The prothrombin time is one measurement reported four ways. British and Irish laboratories report seconds and a ratio; almost everyone reports an INR for anticoagulant monitoring; and much of continental Europe — Germany, Austria, Switzerland, France — reports the Quick value in per cent, where a healthy person sits above 70% and a fully anticoagulated one somewhere near 20%. A clinician handed a discharge summary from the other convention routinely cannot map it, and the two numbers move in opposite directions, which makes the confusion worse than it needs to be.
The awkward part is that only one of these relationships is exact. The PT ratio is the patient’s clotting time divided by the laboratory’s mean normal, and the INR is that ratio raised to the power of the thromboplastin’s International Sensitivity Index. Both are arithmetic. The Quick value is not: it is read off a calibration curve the laboratory builds by diluting normal plasma to 100, 50, 25, 12.5 and 6.25% of its original strength and measuring the clotting time of each dilution. The patient’s result is interpolated onto that curve, and the curve is hyperbolic, so equal steps in seconds are not equal steps in per cent. Every reagent has its own curve. German laboratory guidance is explicit that each manufacturer supplies its own conversion and that the Quick value is not comparable between laboratories — which is the reason the INR was introduced in the first place.
So this page does not offer a universal formula, because there is not one. It asks for the two anchors that define your own laboratory’s curve — the clotting time of undiluted normal plasma and the clotting time of the 1:1 dilution — and fits the standard hyperbolic form through them. That reproduces the correspondences the published tables quote: an INR of about 1.2 lands near Quick 70%, an INR of 2.0 near Quick 29%, an INR of 3.0 near Quick 17%. Treat the answer as accurate to roughly ten percentage points, use it to read a report written in the other convention, and never use it to titrate a vitamin K antagonist. For that, use the INR, which is the only one of these four numbers that was calibrated to be compared between laboratories.
One further caution applies to the Quick value specifically. Because the curve is hyperbolic, its responsiveness is wildly uneven: at the normal end, large changes in clotting time produce small changes in per cent, and at the anticoagulated end the reverse. A published audit of prothrombin activity percentage concluded it correlated worse with albumin and total protein than either PT seconds or the INR, predicted liver failure less well than either, and should be abandoned in favour of reporting seconds and the INR. Where you have the choice, report the INR.
Frequently asked questions
What does a Quick value of 70% mean in INR?
Roughly an INR of 1.2, which is the upper limit of normal. The correspondence is approximate: the Quick value is interpolated from a reagent-specific dilution curve, so the same plasma gives different Quick values in different laboratories while giving the same INR. Use the INR for any decision about anticoagulant dosing.
Can Quick % be converted to INR exactly?
No. The Quick value is read off a calibration curve built by diluting normal plasma, and that curve belongs to the particular thromboplastin reagent. There is no reagent-independent formula, which is exactly why the World Health Organization introduced the INR. Manufacturers supply their own conversions for their own reagents.
Why does the Quick value fall when the INR rises?
They are opposite scales for the same thing. The Quick value expresses coagulation activity as a percentage of normal, so it falls as clotting is impaired. The INR expresses the clotting time as a normalised ratio, so it rises. A patient anticoagulated to an INR of 2.5 typically reports a Quick value near 20 to 25%.
What is a normal Quick value?
Above 70% in most European laboratories, corresponding to an INR of about 1.2 or below. Local reference ranges vary with the reagent, so confirm against the range printed on the report rather than this figure.
Is the Quick value still used?
Widely, across German-speaking Europe and France, but it is being displaced. Its hyperbolic calibration curve makes it unevenly responsive, and published comparisons find it performs worse than PT seconds or the INR as a marker of hepatic synthetic function. Where both are available, the INR is the number to act on.
Related calculators
References
- Kirkwood TB. Calibration of reference thromboplastins and standardisation of the prothrombin time ratio. Thromb Haemost. 1983;49(3):238–244.
- Horsti J, Uppa H, Vilpo JA. Poor agreement among prothrombin time international normalized ratio methods: comparison of seven commercial reagents. Clin Chem. 2005;51(3):553–560.
- Poller L. International Normalized Ratios (INR): the first 20 years. J Thromb Haemost. 2004;2(6):849–860.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.