aPTT (activated partial thromboplastin time)
Clotting test that measures how many seconds a blood sample takes to clot
Also known as
Definition
The aPTT measures how many seconds a blood sample takes to form a clot in the lab. It checks a specific group of clotting factors, proteins made mostly by the liver. It is usually ordered together with the prothrombin time (PT/INR), which covers other clotting factors. Taken together, the two values give a first overview of how your blood clots.
| Parameter | Value |
|---|---|
| Unit | seconds (s) |
| Reference Range | Typically about 25–35 s in adults. Labs use different methods and their ranges differ; the reference range on your own report is what counts. Discuss any deviation with your doctor. |
Agree on your personal target range with your doctor.
↓ What a low value means
A shortened aPTT means the sample clotted faster than usual. This can be linked to high levels of clotting factors, for example during inflammation, in pregnancy or with some advanced cancers, and rarely to the early stage of disseminated intravascular coagulation (DIC). An unusual value does not automatically mean a disease, because clotting in the test tube can differ from clotting in your body. Discuss the result with your doctor.
↑ What a high value means
A prolonged aPTT can be linked to clotting factors that are missing, low or not working properly, for example in hemophilia A or B or von Willebrand disease. Other possible links include liver disease, vitamin K deficiency, problems absorbing nutrients, certain leukemias and anticoagulant medicines such as heparin. Lupus anticoagulant prolongs the aPTT in the test but is associated with increased clotting in the body. Interpretation usually needs further tests; discuss it with your doctor.
When to test
Typical reasons are unexplained bleeding or bruising, a known tendency to bleed before surgery, blood clots, repeated miscarriages, liver disease and monitoring of heparin treatment. No special preparation is needed. Tell your doctor which medicines and herbal remedies you take, and do not stop anything without medical advice.
Frequently asked questions
What is the difference between aPTT and PT/INR? +
Both tests measure how long a blood sample takes to clot, but they cover different groups of clotting factors. That is why they are often ordered together and interpreted side by side.
Why is the aPTT sometimes checked before surgery? +
The test can be useful before a procedure if you have a known tendency to bleed, such as frequent nosebleeds or bruising easily. Whether you need it is up to your doctor.
Does a prolonged aPTT always mean a bleeding tendency? +
No. Anticoagulants such as heparin prolong the aPTT as expected. Lupus anticoagulant prolongs it in the lab but is associated with increased clotting in the body. Clotting in the test tube can also differ from clotting in the body. Whether further tests are needed is something to clarify with your doctor.
Do I need to fast for an aPTT test? +
No special preparation is needed. It is important to tell your doctor which medicines and herbal remedies you take, because some of them can affect the result.
aPTT (activated partial thromboplastin time)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.