aPTT

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.

• 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

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.

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.

• 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.

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.

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.

• 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.

• NVKC, allesovertesten.nl: aPTT
• NVKC, allesovertesten.nl: vWf (Von Willebrand-factor)
• FMS Richtlijn Antitrombotisch beleid: Effect ongefractioneerde heparine
• FMS Richtlijn Antitrombotisch beleid: Hoe meten van DOAC-spiegels