The activated partial thromboplastin time (aPTT) is a blood test that measures how long it takes for blood to form a clot. Normally, when one of your blood vessels is damaged, proteins in your blood called clotting factors come together in a certain order to form blood clots and quickly stop bleeding. The aPTT test can be used to look at how well those clotting factors are working. The test is often used with other tests that monitor clotting factors.

During the test, a healthcare provider will take a blood sample and add a substance that triggers the clotting process. The time it takes for the blood to clot is then measured in seconds. A normal range for aPTT is around 21 to 35 seconds, but test results will vary depending on equipment and methods used. If the aPTT takes longer than normal, it may mean several things, and often other tests are done at the same time as the aPTT to better find out which factors are involved. If the test shows an abnormally short clotting time, it may be a sign of increased risk for blood clots (thrombosis), bleeding, or multiple miscarriages.

The aPTT test is often used to monitor the effectiveness of heparin therapy, a medication used to prevent or treat blood clots. If the test is done because of heparin therapy, the healthcare provider will often want the aPTT to be about 2 to 2.5 times as long as what it would be normally.

Other tests that may be done to measure how well blood is clotting include the prothrombin time (PT), thrombin time (TT), and activated whole blood clotting time (ACT).