Factor VIII (Antihemophilia Factor A)
Does this test have other names?
Antihemophilia factor A test, AHF, factor VIII:C, coagulation factor test
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. Von Willebrand disease is
the most common of all bleeding disorders. In hemophilia A, blood does not 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
If you are low on factor VIII proteins, you may have hemophilia A. Almost all people
with hemophilia A are male. Women are generally only carriers of the gene and have
a 50% chance of passing it on to each of their children. But it's possible for females
to develop the 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 have 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 or dental procedures
Slow wound healing because of repeated bleeding or infection
Joint pain and stiffness
Swollen, hot joints or deformities
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
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
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?
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
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 percent, you may have hemophilia
A, but how severe your risk of bleeding depends on what percentage you have.
If you have normal to decreased level of factor VIII, you may have von Willebrand
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 requires a blood sample, which is drawn through a needle from a vein in your
Does this test pose any risks?
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.
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 like 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 illicit drugs you may use.