Content Map Terms
Partial thromboplastin time (PTT) is a blood test that measures the time it takes your blood to clot. A PTT test can be used to check for bleeding problems.
Blood clotting factors are needed for blood to clot (coagulation). The partial thromboplastin time is an important test because the time it takes your blood to clot may be affected by:
- Blood-thinning medicine, such as heparin. Another test, the activated partial thromboplastin time (APTT) test, may be used to find out if the right dose of heparin is being used.
- Low levels of blood clotting factors.
- A change in the activity of any of the clotting factors.
- The absence of any of the clotting factors.
- Other substances, called inhibitors, that affect the clotting factors.
- An increase in the use of the clotting factors.
Another blood clotting test, called prothrombin time (PT) or INR (international normalized ratio), measures other clotting factors. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems caused by a problem with the clotting factors.
Why It Is Done
Partial thromboplastin time (PTT) is done to:
- Find a cause of abnormal bleeding or bruising.
- Check for low levels of blood clotting factors. The lack of some clotting factors can cause bleeding disorders such as hemophilia.
- Check for conditions that cause clotting problems. Conditions such as antiphospholipid antibody syndrome or lupus anticoagulant syndrome develop when the immune system makes antibodies that attack blood clotting factors. This can cause the blood to clot easily in veins and arteries.
- Check if it is safe to do a procedure or surgery that might cause bleeding.
- Check how well the liver is working.
The activated partial thromboplastin time (APTT) test is used after you take blood-thinners to see if the right dose of medicine is being used. If the test is done for this purpose, an APTT may be done every few hours. When the correct dose of medicine is found, you will not need so many tests.
How To Prepare
Many medicines can change the results of this test. Be sure to tell your doctor about all the non-prescription and prescription medicines you take.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
How It Feels
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
- A longer-than-normal PTT or APTT can mean a lack of or low level of one of the blood clotting factors or another substance needed to clot blood. This can be caused by bleeding disorders, such as hemophilia or von Willebrand's disease.
- A longer-than-normal PTT or APTT can be caused by liver disease, kidney disease (such as nephrotic syndrome), or treatment with blood thinners.
- A longer-than-normal PTT may be caused by conditions such as antiphospholipid antibody syndrome or lupus anticoagulant syndrome. These conditions happen when the immune system makes antibodies that attack blood clotting factors. This can cause the blood to clot easily in veins and arteries.
- The PTT can get longer when you are using heparin, so your PTT value needs to be closely checked. If you have a longer PTT, you may have a higher risk of bleeding.
Current as of: September 8, 2022
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine
Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.
HealthLinkBC Files are easy-to-understand fact sheets on a range of public health and safety topics including disease prevention and immunizations.
Find Services and Resources
If you are looking for health services in your community, you can use the HealthLinkBC Directory to find hospitals, clinics, and other resources.