Blood draw
aPTT
Measures the aPTT, a clotting time that tests part of the clotting factors. For people who bruise easily or bleed for a long time.
€ 21,00
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Other analyses · Blood
aPTT
A clotting time that mainly tests part of the clotting factors; used when investigating a bleeding tendency.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
aPTT: what is measured?
The aPTT (activated partial thromboplastin time) measures in seconds how long it takes for blood plasma to form a clot in the laboratory, after substances that start clotting have been added. Blood clotting is a chain of reactions between clotting factors: proteins that are mainly made in the liver. The aPTT mainly tests factors VIII, IX, XI and XII and the final steps of clotting. A longer time means the plasma clots more slowly in the tube; that does not always mean that someone bleeds more easily in daily life.
What can it detect?
- A shortage of the clotting factors the aPTT measures, such as in hemophilia (hereditary bleeding disorder) or sometimes in von Willebrand disease
- Lupus anticoagulant: antibodies that prolong the aPTT in the tube, but actually increase the risk of thrombosis in the body
- The effect of heparin and some other anticoagulants
- Reduced production of clotting factors in severe liver disease
Result
What does the result mean?
aPTT too high
A prolonged aPTT means that the plasma clots more slowly in the laboratory than expected. According to the Dutch Society for Clinical Chemistry (NVKC), this is usually due to a shortage of one or more clotting factors, an inhibitor of clotting or anticoagulants such as heparin; dabigatran and, to a lesser extent, coumarins also prolong the aPTT. A hereditary shortage, such as hemophilia, causes symptoms such as bruising easily, prolonged bleeding after a small wound, dental treatment or surgery, and heavy blood loss during periods. Lupus anticoagulant and a shortage of factor XII do prolong the aPTT, but do not cause bleeding. With a slightly prolonged aPTT without symptoms, the blood draw itself can also be the cause; on repeat testing the value is then often normal.
aPTT too low
A shortened aPTT usually has no meaning. According to the Dutch Society for Clinical Chemistry (NVKC), a shorter time fits a temporarily raised amount of clotting factor VIII, for example after inflammation or injury; factor VIII is also higher during pregnancy. A difficult blood draw, in which clotting already starts in the tube, can also shorten the time. A shortened aPTT does not cause symptoms by itself and, as a single value, is not a reliable measure of the risk of thrombosis.
After an abnormal result
With a prolonged aPTT, the GP repeats the test, usually together with the prothrombin time, fibrinogen and platelets, and asks about medicines and bleeding symptoms. If the aPTT stays prolonged or there is a bleeding tendency, referral follows to an internist-hematologist or hemophilia treatment center to measure the individual clotting factors and lupus anticoagulant.
Reliability
How reliable is the test?
The aPTT is a global test: it shows that clotting in the tube is slower or faster, but not why. With a bleeding tendency, the aPTT is assessed together with other clotting tests, such as the prothrombin time, fibrinogen and the platelet count. A normal aPTT does not rule out a mild bleeding disorder; von Willebrand disease and disorders of platelet function often give a normal aPTT. The result depends on the reagent used, so reference values differ between laboratories.
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.
- Heparin and dabigatran prolong the aPTT strongly, coumarins and other DOACs to a varying degree; a normal aPTT does not show whether a DOAC is working.
- A high amount of factor VIII, for example with inflammation or during pregnancy, shortens the aPTT and can hide a mild shortage of another factor.
Who it is for
Who is the test useful for?
Useful for
Useful with a bleeding tendency, such as bruising easily, prolonged bleeding after a small wound, dental treatment or surgery, or heavy blood loss during periods, and with a bleeding disorder in the family; usually together with the prothrombin time. Also with severe liver disease. In people without bleeding symptoms and without a bleeding disorder in the family, a single aPTT says little, also as preparation for surgery. People who use heparin are monitored through their prescriber; this test does not replace that.
How common is it?
There are no Dutch figures on how often an abnormal aPTT occurs. Hemophilia is rare and occurs almost only in boys and men, in about 1 in 5,000 to 10,000. Von Willebrand disease is the most common hereditary bleeding disorder, but many people with a mild form have few symptoms and a normal aPTT.
Estimate from professional knowledge; no Dutch registration figure checked
Drawbacks and risks
- A slightly prolonged aPTT due to the blood draw or lupus anticoagulant can cause needless worry about a bleeding tendency and lead to repeated testing.
- A normal aPTT can be falsely reassuring with bleeding symptoms, because mild bleeding disorders such as von Willebrand disease often give a normal aPTT.
Get tested
aPTT: test panels
aPTT 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