Rheumatoid factor (or RF) is an antibody made against a part of an antibody (IgG). It is essentially an anti-antibody. It is often measured to help diagnose Rheumatoid arthritis, although it is present in other conditions also. These other conditions include myositis, lupus (SLE), Sjogrens syndrome and connective tissue diseases.
A positive RF test does not diagnose Rheumatoid arthritis (RA). About 1 out of 20 people have a positive RF test. When a titer RF test is done, a high titer is sometimes associated with RA severity, but not consistently.
The RF test is most helpful when a person has a high chance of being diagnosed with Rheumatoid arthritis, Sjogrens or cryoglobulinemia. These conditions have typical features using xray, lab tests and the clinical exam.
An otherwise normal person with joint pains would not find a test for RF very helpful.