A factor VIII assay tests for a protein involved in blood clotting. If you produce less of this protein, it may indicate a bleeding disorder. Some people may also have high levels of this coagulation factor for various reasons.

A factor VIII assay is a test to determine whether your body produces a typical amount of this particular coagulation factor. Your body needs factor VIII to form blood clots.

Each time you bleed, this protein 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.

Read on to learn more about the test, how to prepare, and what the results may mean.

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

An inherited factor VIII deficiency indicates a condition known as hemophilia A.

If a person has an inherited factor deficiency, other close relatives may be tested to check if they are carriers of a particular gene or if they also have the condition.

Hemophilia A mostly affects 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 people with only one X chromosome will always have hemophilia A if they have this defective gene.

Females have two X chromosomes. So if you have only one X chromosome with the defective gene, your body can still create enough factor VIII. Both X chromosomes would have to have the defective gene for a person with two X chromosomes 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.

Tell your doctor if you’re taking any blood-thinning medications, such as:

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 professional takes a sample of blood from your arm. First, the site is cleaned with an alcohol swab.

Then the healthcare professional 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% 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 due to:

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.

This condition is known as phlebitis, and treatment can include 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 tell you and your family how to look out for signs that you urgently need an infusion of factor VIII.

Depending on the severity of your condition, you may be able to administer a certain form of factor VIII at home following your doctor’s guidance.

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.