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Rheumatoid Factor (Blood)

Does this test have other names?

RF blood test

What is this test?

This test measures the level of a substance called rheumatoid factor (RF) in your blood. It helps your doctor find out whether you have rheumatoid arthritis (RA).

RF is an autoantibody that responds to inflammation caused by RA. Antibodies increase in your blood when they detect a foreign substance, such as bacteria. Autoantibodies, on the other hand, attack your own body's proteins.

RF is linked to chronic, or ongoing, inflammation. So, it may be higher if you have RA, which is an inflammatory condition. Although this autoantibody does not directly cause arthritis, it plays a role in increasing inflammation if you have joint damage. RF is found in the blood of 70 to 80 percent of people with RA.  

This test may also help diagnose other autoimmune diseases, such as Sjögren's syndrome.

Why do I need this test?

You may need this test if your doctor suspects that you have RA. Symptoms of RA include:

  • Low-grade fever

  • Extreme tiredness

  • Loss of weight

  • Muscle pain

  • Numbing or tingling sensation in your hands

  • Morning joint stiffness

Arthritis pain often affects finger and toe joints. The knees and shoulders may also be affected by RA.

If RA is not treated, it can severely affect your daily life and can make it hard to walk. It's important to start treatment early on. It can be hard to know the problem is RA and not other inflammatory illnesses, such as polyarthritis. Women are three times more likely than men to have RA. Infections and cigarette smoking may increase the pain of existing RA.

Long-term effects of RA include damage to your cartilage and bones and decreased function in your joints.

What other tests might I have along with this test?

Your doctor may order other blood tests to help diagnose RA. These include:

  • Anti-citrullinated peptide/protein antibody test

  • Antinuclear antibody, or ANA, testing

  • Complete blood count, or CBC

Your doctor may also order X-rays of your wrists, hands, and feet to look for joint damage.

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 milliliter (U/mL). If your level is lower than 60 U/mL, your results are considered negative and you likely don't have RA. Levels above that may mean that you have RA or another autoimmune disease.

The normal level for an older adult may be slightly higher than 60 U/mL.

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?

Certain infections can raise your level of RF.  

How do I get ready for this test?

You don't need to prepare for this test.