The Rheumatoid Factor (RF) test is a blood test that looks for the presence of rheumatoid factor in your blood. Rheumatoid factor is an antibody, a protein produced by your immune system. While RF is often associated with rheumatoid arthritis (RA), an autoimmune disease that causes joint inflammation, it can also be present in people with other conditions or even in healthy individuals.   Why is the RF test done? Help diagnose rheumatoid arthritis: Although not solely diagnostic, a positive RF test combined with other clinical findings and tests (like anti-CCP antibodies) can help confirm an RA diagnosis. Assess the severity of RA: Higher RF levels may sometimes indicate more severe disease.   Diagnose other autoimmune diseases: Such as Sjogren's syndrome, lupus, and certain infections.