What is the APTT test?

The APTT (activated partial thromboplastin time) is a measure of one part of the clotting system known as the intrinsic pathway. This pathway involves a number of coagulation factors, which are proteins involved in the normal clotting process.

The APTT is used to measure the effects of treatment with intravenous (IV) heparin therapy, to ensure that the blood is not thinned too much or too little.

How an APTT test performed?

APTT is a blood test. It requires a few millilitres of blood from a vein, and the tube must be filled to the correct level to avoid a false reading.

For IV heparin therapy, the APTT is usually checked approximately 6 hours after the start of treatment, once the effect of heparin treatment has stabilised. Adjustments to the dose of the heparin infusion are then made, based on the APTT result.

Why you would need the APTT test

Conditions which may be treated with IV Heparin that require an APTT test include:

  • Deep vein thrombosis (DVT)
  • Pulmonary embolism (PE)
  • Unstable angina (now more commonly known as acute coronary syndrome or ACS)
  • Atrial fibrillation (AF)

These conditions are commonly treated with subcutaneous low-molecular weight heparin, but IV heparin may be preferred in some patients who are at increased risk of bleeding, because it is easier to stop or reverse the anticoagulation (blood-thinning).

The APTT does not measure the effect of treatment with low-molecular weight heparin injections.

The APTT may also be checked as part of the coagulation profile when investigating easy bruising or bleeding, or when haemophilia is suspected.

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APTT test results explained

APTT is the time in seconds for a specific clotting process to occur, in the laboratory test. This result is always compared to a control sample of normal blood. If the test sample takes longer than the control sample, it indicates decreased clotting function in the intrisic pathway.

IV heparin therapy usually aims for a range of APTT, eg 45 to 70 seconds, but may also be expressed as a ratio of test to normal, eg 1.5 times normal.

A high APTT in the absence of heparin treatment can be due to haemophilia, which may require further testing.

Related specialists

Related procedures

  • Blood Test (venesection)
  • Intravenous Cannulation

Related tests

Also known as

  • Activated partial thromboplastin time
  • PTT



This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.

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