What Is the APTT Test?
The APTT (activated partial thromboplastin time) tests the intrinsic clotting pathway. It measures how long it takes blood to clot when specific clotting factors are activated. It is mainly used to monitor heparin anticoagulation therapy and investigate unexplained bleeding.
Normal APTT Range
| Situation | APTT Range |
|---|---|
| Normal (not on anticoagulants) | 25–35 seconds |
| Therapeutic (on heparin) | 60–100 seconds (or 1.5–2.5× normal) |
Causes of Prolonged APTT
- Heparin therapy (intended effect)
- Haemophilia A (Factor VIII deficiency) or Haemophilia B (Factor IX deficiency)
- Von Willebrand disease
- Lupus anticoagulant (antiphospholipid syndrome)
- Liver disease
- Disseminated intravascular coagulation (DIC)
Causes of Short APTT
A shorter-than-normal APTT is less clinically significant but can be seen in early DIC or in patients with an acute thrombotic event.
APTT vs PT — What's the Difference?
PT/INR tests the extrinsic pathway and monitors warfarin. APTT tests the intrinsic pathway and monitors heparin. Together they give a complete clotting picture.
FAQs
Do I need to fast before an APTT test?
No fasting is required.
Can APTT be normal with haemophilia?
Mild haemophilia may show a borderline or normal APTT — specific factor assays are then ordered.
What does a very long APTT (>100 sec) mean?
This usually indicates over-anticoagulation with heparin or a serious factor deficiency requiring urgent review.
Medical Disclaimer: APTT results must always be interpreted in clinical context by a doctor, particularly when monitoring anticoagulation therapy.