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

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

A rheumatoid factor test is one of a group of blood tests primarily 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 doctor choose the treatment approach that will work best for your situation.

During a rheumatoid factor test, a small sample of blood is drawn from a vein in your arm. This typically takes just a few minutes. Your blood sample is sent to a laboratory for testing.

A positive rheumatoid factor test result indicates that a high level of rheumatoid factor was detected in your blood. A higher level of rheumatoid factor in your blood is closely associated with autoimmune disease, particularly rheumatoid arthritis. But a number of other diseases and conditions can raise rheumatoid factor levels, including:

• Cancer
• Chronic infections
• Inflammatory lung diseases, such as sarcoidosis
• Mixed connective tissue disease
• Sjogren's syndrome
• Systemic lupus erythematosus

Some healthy people — particularly older individuals — 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.