Your doctor may recommend a factor VIII assay test to determine whether your body produces an appropriate level of this particular coagulation factor. Your body needs factor VIII to form blood clots.

Each time you bleed, it triggers a series of reactions known as the “coagulation cascade.” Coagulation is part of the process your body uses to stop blood loss.

Cells called platelets create a plug to cover the damaged tissue, and then certain types of your body’s clotting factors interact to produce a blood clot. Low levels of platelets or any of these necessary clotting factors can prevent a clot from forming.

This test is typically used to help determine the cause of prolonged or excessive bleeding.

Your doctor may recommend the test if you have a family history of bleeding disorders or if you have experienced any of the following symptoms:

This test may also be ordered as part of a coagulation factor assay that checks the amounts of multiple types of coagulation factors. Your doctor may recommend this test if they believe you have an acquired or hereditary condition that’s causing your bleeding disorder, such as:

This assay can also help to determine whether you have inherited factor VIII deficiency, particularly if you’ve been experiencing bleeding episodes since childhood.

If a family member has an inherited factor deficiency, other close relatives may be tested to help confirm a diagnosis.

An inherited factor VIII deficiency is called hemophilia A.

This hereditary condition mostly affects only males because it’s linked to a defective gene on the X chromosome and is inherited in an X-linked recessive manner. This means that men, who only have one X chromosome, will always have hemophilia A if they have this defective gene.

Females have two X chromosomes. So if a woman has only one X chromosome with the defective gene, their body can still create enough factor VIII. Both X chromosomes would have to have the defective gene for a woman to have hemophilia A. This is why hemophilia A is rare in females.

If you’ve already been diagnosed with a factor VIII deficiency and are undergoing treatment, your doctor may order this test to determine the effectiveness of your treatment.

No special preparation is necessary for this test. You should tell your doctor if you’re taking any blood-thinning medications, such as warfarin (Coumadin), enoxaparin (Lovenox), or aspirin.

Depending on the coagulation factors that may be tested with your factor VIII assay, your doctor may advise you to stop taking blood thinners before the test.

To perform the test, a healthcare provider takes a sample of blood from your arm. First, the site is cleaned with an alcohol swab.

Then the healthcare provider inserts a needle into your vein and attaches a tube to the needle to collect blood. When enough blood has been collected, they remove the needle and cover the site with a gauze pad.

The blood sample is then sent to a laboratory for analysis.

Normal result

A normal result for a factor VIII assay should be around 100 percent of the laboratory reference value, but keep in mind that what is considered a normal range may vary from one lab assay to the next. Your doctor will explain the specifics of your results.

Abnormal result

If you have an abnormally low level of factor VIII, it could be caused by:

If you have an abnormally high level of factor VIII, it may be caused by:

As with any blood test, there’s a slight risk of bruising or bleeding at the puncture site. In rare cases, the vein may become inflamed and swollen after blood is drawn.

Such a condition is known as phlebitis and can be treated by applying a warm compress several times a day.

Ongoing bleeding could also be a problem if you have a bleeding disorder or are taking blood-thinning medication, such as warfarin, enoxaparin, or aspirin.

If you’ve been diagnosed with a factor VIII deficiency, your doctor will prescribe replacement concentrates of factor VIII. The amount you require will depend on:

  • your age
  • your height and weight
  • the severity of your bleeding
  • the site of your bleeding

To help prevent a bleeding emergency, your doctor will teach you and your family when it’s urgent that you have an infusion of factor VIII. Depending on the severity of hemophilia A a person has, they may be able to administer a certain form of factor VIII at home after receiving instruction.

If your levels of factor VIII are too high, you are likely at a higher risk for thrombosis, which is blood clot formation in your blood vessels. In this case, your doctor may perform additional tests or prescribe anticoagulant therapy.