Factor VIII Activity

Coagulation factor tests are blood tests that check one or more of your clotting factors. A Factor VIII activity test is often ordered to look into a prolonged aPTT (a screening clotting test) or as part of an evaluation for von Willebrand disease, often alongside other tests such as von Willebrand factor and Ristocetin cofactor activity.

Your liver makes most of your clotting factors. But normally, clotting factors are turned off, so you don't form abnormal blood clots. When you have an injury that causes bleeding, blood cells called platelets become stickier, so they can stick to each other to make a clot and stop the bleeding.

The platelets release molecules into your blood that begin to turn on the clotting factors. The clotting factors work together in a chain reaction to form a blood clot that will stay firmly in place.

Factor VIII is one of the clotting factors. A shortage of Factor VIII activity is the cause of hemophilia A, an inherited bleeding disorder that mainly affects males. Women who carry the gene that causes hemophilia A may have somewhat lower Factor VIII activity than usual.

Your provider may need to order other tests to find the cause of a problem with your clotting factors.

Lower than normal levels of one or more clotting factors or a missing clotting factor may mean you have a bleeding disorder. Depending on which clotting factors were tested, your results may show the type of bleeding disorder you have and how serious it is.

Higher than normal levels of one or more clotting factors may mean you have a disorder that makes your blood clot more than it should. Your provider may recommend medicine and heart-healthy lifestyle changes to help prevent clots. You may also need to avoid hormone replacement therapy for menopause and birth control pills with estrogen, because they may increase the risk of blood clots.

Some things can affect Factor VIII results. Levels can go up during illness, infection, or other stress on the body, so a result may not reflect your usual baseline. Lupus anticoagulant antibodies can make Factor VIII levels look falsely low, and antibodies against Factor VIII (which can develop on their own or after replacement therapy) can also drive results lower. The test is not recommended if you're being treated with emicizumab (Hemlibra), because that medicine causes falsely high results. Talk with your provider to find out what your test results mean and what treatment is best for you.