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
Prothrombin time (PT)
A clotting time that tests the clotting factors made by the liver; used with a bleeding tendency, liver disease and vitamin K deficiency.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
Prothrombin time (PT): what is measured?
The prothrombin time (PT) measures how long it takes for blood plasma to form a clot in the laboratory, after tissue factor (thromboplastin) has been added. The test is mainly sensitive to clotting factors II, V, VII and X and to fibrinogen. These are made in the liver, and factors II, VII and X need vitamin K. The laboratory that performs this test gives the result as a Quick value in percent of normal: the lower the percentage, the more slowly the blood clots. If the prothrombin time is given in seconds, it is the other way around: a longer time means slower clotting. The INR is calculated from the same measurement.
What can it detect?
- A shortage of clotting factors from the liver, with a bleeding tendency
- Reduced production capacity of the liver in severe liver disease
- Vitamin K deficiency, for example due to poor absorption of fat from food or long-term antibiotic use
- The effect of coumarins (acenocoumarol, phenprocoumon) and, to a varying degree, of some DOACs
Result
What does the result mean?
Prothrombin time (PT) too high
A high Quick value means that the plasma clots quickly in the tube. This almost never has any meaning: it is not a sign of an increased risk of thrombosis and it does not cause symptoms. A slightly raised Quick value is not followed by further testing. If a laboratory gives the prothrombin time in seconds, a high value actually means slower clotting, as described for a low Quick value.
Prothrombin time (PT) too low
A low Quick value, or a prolonged prothrombin time in seconds, means that the blood clots more slowly than normal. The most common cause is the use of a coumarin (acenocoumarol or phenprocoumon); in that case a low Quick value is exactly what is intended. Rivaroxaban and edoxaban can also lower the value. In someone without anticoagulants, according to the Dutch Society for Clinical Chemistry (NVKC), a low value fits a bleeding tendency, liver disease in which the liver makes too few clotting factors, or vitamin K deficiency; very rarely there is a congenital shortage of a clotting factor. Symptoms that may go with it are bruising easily, nosebleeds, prolonged bleeding and blood in the urine or stool.
After an abnormal result
With a low Quick value without anticoagulants, the GP repeats the test, often together with the aPTT and fibrinogen, and looks at liver function, vitamin K absorption and medicines. If clotting stays slow without a clear cause, or there are bleeding symptoms, referral to an internist follows to measure the individual clotting factors. In coumarin users, the INR measurements of the anticoagulation clinic are what counts.
Reliability
How reliable is the test?
The prothrombin time is a global test: an abnormal result shows that clotting is slower, but not why. According to the Dutch Society for Clinical Chemistry (NVKC), with a bleeding tendency the test is assessed together with the aPTT. The result in seconds or percent depends on the reagent and differs between laboratories; for monitoring coumarins the INR is therefore used, which is calculated from the same measurement. A normal prothrombin time does not rule out a bleeding disorder: hemophilia and von Willebrand disease give a normal prothrombin time.
What can affect the result?
- Clotting tests are sensitive to how the blood is drawn: a tourniquet left on too long, a difficult draw or a tube that is not filled exactly to the line can disturb the result.
- The plasma must be separated within a few hours of the blood draw and frozen if needed; with delayed transport the result is less reliable.
- Coumarins lower the Quick value strongly, rivaroxaban and edoxaban to a varying degree and apixaban hardly at all; so a normal result does not show whether a DOAC is working.
- In coumarin users, antibiotics, illness with fever or diarrhea, alcohol and changes in vitamin K from food change the result within a few days.
Who it is for
Who is the test useful for?
Useful for
Useful with a bleeding tendency, such as bruising easily, nosebleeds or prolonged bleeding, usually together with the aPTT, and with severe liver disease or a suspected vitamin K deficiency. In healthy people without symptoms, a single value says little. People who use a coumarin have their INR checked through the anticoagulation clinic (trombosedienst); this test does not replace that check, because the anticoagulation clinic sets the dose based on its own measurements and the result of this test is only known after a few days.
How common is it?
There are no Dutch figures on how often an abnormal prothrombin time occurs in people without anticoagulants. In that case it is usually due to liver disease or vitamin K deficiency; a congenital shortage of one of the measured clotting factors is very rare. Much more often, a low Quick value is the intended effect of a coumarin.
NVKC, allesovertesten.nl; no Dutch registration figure available
Drawbacks and risks
- People who use a coumarin and adjust their own dose based on this result instead of through the anticoagulation clinic run a risk of bleeding or thrombosis.
- A slightly abnormal value due to the blood draw or transport can cause needless worry and lead to repeated testing.
Get tested
Prothrombin time (PT): test panels
Prothrombin time (PT) 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.
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