Activated Coagulation Time

ACT, activated clotting time

This test measures how long it takes your blood to clot. It's often used to check how well a medicine called heparin is working. Heparin slows the ability of blood to clot, and the ACT test helps your healthcare provider find the right dosage. This test uses whole blood, which is different from the standard tests for coagulation. Those tests use plasma. Plasma is blood that has had the red cells, white cells, and platelets removed. The ACT test is usually used during procedures when results may be needed right away.

You may need this test if you are getting heparin to prevent your blood from clotting during a procedure such as open heart surgery, cardiac catheterization, or kidney dialysis. This test is usually done in the hospital at your bedside.

You may have other blood tests to measure how quickly your blood clots. A blood test called activated partial thromboplastin time (APTT) is often done as a backup to the results of the ACT test.

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

This test doesn't measure the amount of heparin in your blood. It measures only how long it takes for a blood clot to form. The results are given in the number of seconds:

• 70 to 120 seconds is the usual amount of time for blood to clot without heparin. 

• 180 to 240 seconds is the usual amount of time for blood to clot with heparin. This is called the therapeutic range.

The therapeutic range is usually about double the normal clotting time. It's important to be in this range because if your ACT is too low, you may be in danger of a blood clot forming during a procedure. Some procedures may require an even longer ACT.

If your ACT is too high, you may be in danger of bleeding.

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

Other factors that may affect your results include:

• The effects of surgery

• Body temperature

• Other medicines you are taking

• Getting IV (intravenous) fluids, which can dilute your blood

• Platelet counts and platelet function

• Coagulation factor deficiencies

You don't need to prepare for this test. But be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.