Factor VIII (Antihemophilia Factor A)

Does this test have other names?

Antihemophilia factor A test, AHF, factor VIII:C, coagulation factor test, factor assay

What is this test?

This test measures the activity of factor VIII, a blood-clotting protein. The test can find out whether you have hemophilia A or another clotting disorder.

Hemophilia A is the most common severe bleeding disorder. In hemophilia A, blood doesn't clot as it should. This puts a person at risk of uncontrolled bleeding. Under normal circumstances, certain proteins, including factor VIII, come together to form blood clots and quickly stop bleeding.

If your factor VIII protein levels are low, you may have hemophilia A. Almost all people with hemophilia A are male. Women are generally only carriers of the gene and have a 1 in 2 chance of passing it on to each of their children. (Boys who receive the gene would be affected and girls would be carriers.) But it's possible for females to develop mild symptoms of this disorder.

Because blood-clotting proteins work together to stop bleeding, the test may be done as part of an overall screening for the proteins involved in clotting. 

Why do I need this test?

You may need this test if you have a family history of the disorder or if you have one or more of these symptoms:

• Prolonged and unexplained bleeding after minor cuts, surgeries, or dental procedures

• Easy bruising

• Slow wound healing because of repeated bleeding or infection

• Joint pain and stiffness

• Swollen, hot joints or deformities

• Heavy menstruation

• Severe low back pain, usually on one side

• The need for a blood transfusion after a minor injury

You may also have this test if you have abnormal results from other blood tests that measure how well your blood clots. 

What other tests might I have along with this test?

Tests called partial thromboplastin time (PTT) and prothrombin time (PT) are usually the first step in hemophilia testing. These tests focus on clotting pathways. If you haven't already had these tests, you may have them along with your factor VIII test. Even if you have hemophilia, the results of your PTT and PT tests may be normal, so the blood test for factor VIII is used to confirm the diagnosis.

In addition, you will probably have your platelet count measured. This is part of a routine test called a complete blood count.

You may also have other procedures, including the factor VIII antigen assay. This is a separate test to find out the actual amount of factor VIII in your blood, not its clotting activity. Your healthcare provider may also order a mixing study and a factor VIII inhibitor test. These look for antibodies in the blood that could deactivate factor VIII.

You may also have a von Willebrand factor test. The von Willebrand factor is a protein that "glues" platelets together to help form a clot. It protects factor VIII from breaking down.

If you are female and have a family history of hemophilia, your healthcare provider may order molecular genetic testing to find out whether you are a carrier.  

What do my test results mean?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

Test results are usually reported as a percentage of a "normal" result of 100%. Normal ranges for factor VIII levels are 50% to 150%.

If your factor VIII activity level is less than 50%, you may have hemophilia A, but how severe your risk of bleeding is depends on what percentage you have.

If you have bleeding problems with normal to decreased level of factor VIII, you may have von Willebrand disease.

Your healthcare provider will look at your factor VIII test result along with the results of other tests to better understand what the results mean.

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

Taking aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs) or certain other medicines may affect your test results.

How do I get ready for this test?

You may need to stop taking certain medicines before having the test. These may include NSAIDs, such as aspirin, ibuprofen, or naproxen. 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.