A rheumatoid factor test measures the amount of rheumatoid factor in your blood. Rheumatoid factors are proteins made by your immune system that can attack healthy tissue in the body.

High levels of rheumatoid factor in the blood are most often related to autoimmune diseases, such as rheumatoid arthritis and Sjogren syndrome. But rheumatoid factor may be detected in some healthy people. And sometimes people with autoimmune diseases have normal levels of rheumatoid factor.

A rheumatoid factor test is one of a group of blood tests mainly used to help pinpoint a diagnosis of rheumatoid arthritis. These other tests may include:

• Anti-nuclear antibody (ANA).
• Anti-cyclic citrullinated peptide (anti-CCP) antibodies.
• C-reactive protein (CRP).
• Erythrocyte sedimentation rate (ESR, or sed rate).

The amount of rheumatoid factor in your blood may also help your health care team choose the treatment plan that will work best for you.

During a rheumatoid factor test, a member of your health care team will take a small sample of blood from a vein in your arm. This often takes just a few minutes. Your blood sample is sent to a lab for testing. After the test, your arm might be tender for a few hours, but you'll be able to resume most normal activities.

A positive rheumatoid factor test result shows that you have a high level of rheumatoid factor in your blood. A higher level of rheumatoid factor in your blood is closely linked with autoimmune diseases, especially rheumatoid arthritis. But a number of other diseases and conditions can raise rheumatoid factor levels, including:

• Cancer.
• Chronic infections, such as viral hepatitis B and C.
• Inflammatory lung diseases, such as sarcoidosis.
• Mixed connective tissue disease.
• Sjogren syndrome.
• Systemic lupus erythematosus.

Some healthy people — usually older people — have positive rheumatoid factor tests, though it's not clear why. And some people who have rheumatoid arthritis will have low levels of rheumatoid factor in their blood.

Cigarette smokers also may have positive rheumatoid factors. Smoking is a risk factor for developing rheumatoid arthritis.

Results from a rheumatoid factor test can be hard to understand. An expert should review the results. It's important to discuss the results with a doctor trained in autoimmune and arthritis conditions, called a rheumatologist, and ask them any questions you may have.