Version July 2025
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What is activated clotting time? —
Activating clotting time, or "ACT," is a blood test. It is a way to measure how long it takes blood to clot. It is also called "activated coagulation time."
Your ACT tells your doctor if your blood is clotting normally.
Other tests called "PT" or "PTT" are more often used to measure clotting time. But ACT is sometimes used instead. This is because it gives results almost immediately, while the other tests take longer.
Why might I get an ACT test? —
Doctors mainly use this test for people who need to take a high dose of "unfractionated heparin" for surgery or a procedure. Heparin is a medicine that decreases blood clotting. It is sometimes called a "blood thinner." Your ACT results can help your doctor make sure you are getting the right dose of heparin.
Your doctor might do an ACT test if you need unfractionated heparin for a procedure such as:
●Heart surgery
●Cardiac catheterization
●Hemodialysis
●Heart-lung bypass
●Extracorporeal membrane oxygenation, or "ECMO"
What happens during an ACT test? —
A doctor or nurse uses a needle to take a small amount of blood from your vein. This usually happens in the hospital, before or during your procedure.
Often, you are under anesthesia when they take your blood. Or if you already have an IV (a thin tube that goes into a vein), they might take your blood from there.
Most of the time, getting blood taken does not cause problems. You might have a little soreness or bruising where the needle went in.
What do my results mean? —
Your ACT test result shows how long it took for your blood to clot, for example, 180 seconds. A higher number means your blood is taking longer to clot.
If your ACT is too low or too high, your doctor will adjust your heparin dose as needed, or give medicine to reverse the effects of heparin. They might also do other blood tests to learn more about why your ACT is too high or low.
Patient education: Prothrombin time and INR (PT/INR) (The Basics)
Patient education: Partial thromboplastin time test (The Basics)
Patient education: How to take anticoagulants safely (The Basics)
