Blood draw · 2 analyses
Prothrombin time (PT/INR)
Measures the prothrombin time and INR, a clotting time based on liver clotting factors. For a bleeding tendency, liver disease or vitamin K deficiency.
€ 17,00
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Other analyses · Blood
INR
A calculated measure of clotting time that is comparable between laboratories; mainly used to set the dose of coumarins.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
INR: what is measured?
The INR (International Normalized Ratio) is not a separate measurement but a calculation from the prothrombin time. The laboratory divides the measured prothrombin time by a normal prothrombin time and corrects for the sensitivity of the reagent used (the ISI). This way, every laboratory gives about the same INR for the same clotting. In someone without anticoagulants the INR is around 1; according to the Dutch Society for Clinical Chemistry (NVKC), an INR of 2 means that the blood clots about twice as slowly.
What can it detect?
- Whether the dose of a coumarin (acenocoumarol, phenprocoumon) is in the target range
- An anticoagulant effect that is too strong or too weak, with a risk of bleeding or thrombosis
- Slower clotting due to liver disease or vitamin K deficiency in someone without anticoagulants
Result
What does the result mean?
INR too high
A raised INR means that the blood clots more slowly. In coumarin users this is intended; the Dutch medical specialists' guideline (FMS) on antithrombotic treatment names a target range of 2.0 to 3.0 or, with a higher risk of clots, 2.5 to 3.5. An INR above the target range gives a higher risk of bleeding, such as nosebleeds, bruises, blood in the urine or stool and, rarely, a brain hemorrhage. Antibiotics, illness with fever, diarrhea or vomiting, alcohol and less vitamin K from food can make the INR rise. In someone without anticoagulants, a raised INR fits liver disease or vitamin K deficiency, and rarely a congenital shortage of a clotting factor.
INR too low
In someone without anticoagulants, an INR around 1 is normal and a slightly lower value has no meaning. In coumarin users, an INR below the target range means the blood clots too quickly and protection against a (new) thrombosis or stroke is insufficient. Causes are a missed dose, more vitamin K from food or supplements, or medicines that reduce the effect; according to the Dutch Society for Clinical Chemistry (NVKC), the contraceptive pill and some sleeping pills, among others, can lower the INR. An INR that is too low does not cause symptoms by itself.
After an abnormal result
In coumarin users, the anticoagulation clinic adjusts the dose based on its own INR measurements; with a very high INR, vitamin K can be given. With a raised INR without anticoagulants, the GP repeats the test and checks liver function and vitamin K absorption, with referral to an internist if needed. A high INR with bleeding symptoms calls for a quick assessment by a doctor.
Reliability
How reliable is the test?
The INR was developed for monitoring coumarins. According to the Dutch medical specialists' guideline (FMS), the effect of acenocoumarol and phenprocoumon is measured with the INR, and the time of the last dose must be taken into account when reading the result. In people without anticoagulants, the INR adds little to the prothrombin time itself, except that the result is comparable between laboratories. For DOACs and heparin, the INR is not a suitable measure.
What can affect the result?
- With coumarins, the INR depends on the time of the last dose; the FMS guideline assumes a dose in the evening and a measurement in the morning.
- Antibiotics, fever, diarrhea, alcohol, changes in vitamin K from food or supplements and new medicines change the INR within a few days.
- Clotting tests are sensitive to the blood draw and transport: a difficult draw, a tube that is not filled properly or too long a time before centrifuging can disturb the result.
- Rivaroxaban and edoxaban can raise the INR, but the INR is not a measure of how well a DOAC is working.
Who it is for
Who is the test useful for?
Useful for
The INR is intended for people who use a coumarin, and for assessing clotting with liver disease or vitamin K deficiency. People who use a coumarin have their INR checked through the anticoagulation clinic (trombosedienst) or measure it themselves under the clinic's supervision; this test does not replace that check. The anticoagulation clinic sets the dose based on its own INR measurements, and the result of this test is only known after a few days. For people who use a DOAC or no anticoagulant, the INR says little.
How common is it?
A raised INR occurs mainly in people who use a coumarin; there it is the aim of the treatment. In people without anticoagulants, a raised INR is unusual and usually fits liver disease or vitamin K deficiency.
NVKC, allesovertesten.nl; no Dutch registration figure available
Drawbacks and risks
- An INR result from this test is not a basis for adjusting the dose of a coumarin yourself; a wrong adjustment can lead to bleeding or thrombosis.
- The result is only known after a few days; in coumarin users, the INR may already have changed by then.
Get tested
INR: test panels
INR is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and results