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Factor XI

Does this test have other names?

FXI, factor XI deficiency test, test for hemophilia C

What is this test?

This test measures the amount of factor XI in your blood. Factor XI is a substance that plays an important role in blood clotting.

If you don't have enough factor XI, you may have a condition called factor XI deficiency. This is also known as hemophilia C. Hemophilia C is a bleeding disorder that can range from mild to moderate. If you have hemophilia C, your tendency to bleed is not as severe as that seen in the other types of hemophilia.

Why do I need this test?

You might need this test if your doctor suspects that you have factor XI deficiency.

What other tests might I have along with this test?

Your doctor may order different types of blood-clotting tests. These may include:

  • Activated partial thromboplastin time, or aPTT, test, which shows the activity of several substances involved in blood clotting

  • Prothrombin time, or PT, test, which measures how long it takes your blood to form a clot

  • Platelet count, which is normally done if a bleeding disorder is suspected

  • Thrombin time, which measures how quickly your body makes fibrin, a protein that is part of the clotting process

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 health care provider.

Results are given in units per deciliter (U/dL). The normal range for factor XI activity levels is 70 to 150 U/dL, or 70 to 150 percent.

If your results are lower than normal, it means you may have factor XI deficiency. This condition is quite rare and occurs most often among Ashkenazi Jews.

If your results are less than 15 percent of normal, you may have severe factor XI deficiency. People with factor XI deficiency also usually have a prolonged aPTT along with normal thrombin and prothrombin times.

If your results are 20 to 70 percent of normal, you may have a mild deficiency. The only way to make a sure diagnosis is to do more testing.

Levels outside the normal range may also mean that you have liver disease or a vitamin K deficiency.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

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?

Other factors aren't likely to affect your results.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor 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.