Rheumatoid Factor (Blood)

RF blood test, rheumatoid arthritis (RA) factor

This test measures the level of a substance called rheumatoid factor (RF) in your blood. It helps your healthcare provider determine if you have rheumatoid arthritis (RA).

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

RF is linked to long-term (chronic) 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% of people with RA.

This test may also help diagnose other rheumatic diseases, chronic infections, or autoimmune diseases, such as Sjögren syndrome or lupus.

You may need this test if your healthcare provider 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, make it hard to walk or use your hands, and cause deformities of your joints. 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. Females are two times more likely than males 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.

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

• Cyclic citrullinated peptide, or CCP, antibody test

• Antinuclear antibody, or ANA, testing

• Complete blood count, or CBC

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

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 milliliter (U/mL). If your level is lower than 20 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 20 U/mL.

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

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. 

Certain infections can raise your level of RF.

You don't need to prepare for this test. Be sure your healthcare provider knows 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.