Prolonged APTT: What It Means

The aPTT looks at part of the body's clotting machinery, the intrinsic and common pathways. Lab Tests Online UK explains that it is often used, together with the PT, as a starting place when investigating the cause of a bleeding or clotting episode. If factors in those pathways are lacking or abnormal, or if something inhibits them, the test takes longer.

A prolonged result therefore says that clotting in the test tube is slow. It does not say why, and it does not on its own say that you bleed easily. That is the reason a clinician usually does not act on the number alone: the next question is whether the result is real, and then which of several causes is responsible.

Lab Tests Online UK lists pre-analytical problems as one cause of a prolonged aPTT: an insufficient or clotted sample, the wrong ratio of anticoagulant to blood, and unusually high or low haematocrit. It singles out heparin contamination as the most common problem, particularly when blood is collected from intravenous lines that are kept open with heparin flushes.

It also notes that eating a high-fat meal before the test may interfere with it, and that the blood should be collected from a medium to large vein without a tourniquet being applied for a prolonged time. A repeat sample, taken carefully, is therefore a sensible first step before looking for rare conditions.

Lab Tests Online UK separates the real causes of a prolonged aPTT into factor deficiencies, inhibitors and drugs.

When heparin and sampling problems are excluded, Lab Tests Online UK says the aPTT is followed by mixing studies. The patient's plasma is mixed with pooled normal plasma. If the aPTT corrects, a factor deficiency is likely and specific factor assays follow. If it does not correct, an inhibitor is likely, and further testing looks for antibodies to specific factors or for lupus anticoagulant, for example with a dilute Russell viper venom test.

The same page offers a comparison with the PT. A prolonged aPTT with a normal PT suggests decreased or defective factor VIII, IX, XI or XII, von Willebrand disease or lupus anticoagulant. If both are prolonged, factors I, II, V or X, liver disease or disseminated intravascular coagulation come into view.

It is also used with recurrent miscarriages, which may be associated with anticardiolipin or antiphospholipid antibodies, and as part of an evaluation for antiphospholipid syndrome after a thrombotic episode.

Pre-surgery bloods are a common way for a prolonged aPTT to be discovered in someone who has never bled abnormally. Lab Tests Online UK says the aPTT and PT are sometimes used as pre-surgical screens for bleeding tendencies, though numerous studies have shown they are not useful for this purpose, and that asymptomatic patients are usually only screened if their doctor believes it is necessary.

It adds that the test may be used in a pre-surgical evaluation especially if the surgery carries an increased risk of blood loss or if the person has a clinical history of bleeding such as nosebleeds and bruising. A bleeding history is therefore often as informative as the number, and the anaesthetic or haematology team will usually ask about it.