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 protein 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 a rare bleeding disorder. It 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 may need this test if your healthcare provider believes that you have factor XI deficiency.

What other tests might I have along with this test?

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

• Activated partial thromboplastin time (aPTT) test. This shows the activity of several substances involved in blood clotting.

• Prothrombin time (PT) test.) This measures how long it takes your blood to form a clot.

• Platelet count. This test is normally done if it's believed you may have a bleeding disorder.

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

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.

Results are given in units per deciliter (U/dL). The normal range for factor XI activity levels is 65 to 130 U/dL, or 65% to 130%.

If your results are lower than normal, it means you may have factor XI deficiency. This condition is quite rare and happens most often among people with Ashkenazi Jewish background.

If your results are less than 20% of normal, you may have severe factor XI deficiency. People with factor XI deficiency also often have a prolonged aPTT along with normal thrombin and prothrombin times.

If your results are 20% to 60% 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 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?

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. Tell your healthcare provider 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.