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Rheumatoid Factor

Abnormal Immunoglobulin G (IgG) antibodies are produced by the lymphocytes in the synovial membranes. They act as antigens. Other IgG and IgM antibodies react with these abnormal antigens to produce immune complexes. The reactive IgM is called the Rheumatoid Factor (RF).

In English that means that in people with RA the immune system goes haywire and the process creates some abnormal antibodies.

The RF is a measurement of the reactive IgM antibodies. It is important to note that the exact role of these antibodies in rheumatoid arthritis is not yet fully understood.

Approximately 80% of the people with rheumatoid arthritis have positive titers. The other 20% are considered seronegative which means the have no identifiable RF present.  So, a negative RF does not rule out the existence of RA. The factor may also be found in the presence of other autoimmune diseases and even in a small percentage of healthy people.

Type of Test: Blood Test

What the Results Mean:

The Rheumatoid Factor is usually expressed as titer.

A normal or negative titer is less than 1:20.

Levels between 1:20 and 1:80 may indicate something is going on, but not necessarily rheumatoid arthritis. Other autoimmune diseases such as lupus and Sjogrens syndrome may cause a low positive titer. Heart disease, liver disease, mononucleosis and other conditions may also cause a positive result.

Titers over 1:80 are usually, but not always indicative of rheumatoid arthritis.

Since this is not a definitive test, a diagnosis should never be based solely on the results of the rheumatoid factor.

References:
Common Blood Tests : What Every Patient Must Know About Lab Tests by N. L. Gifford(Editor)
Mosby's Manual of Diagnostic and Laboratory Tests by Kathleen Deska Pagana, Timothy James Pagana.