RHEUMATOID FACTOR

CLINICAL UTILITY:

Rheumatoid factors are autoantibodies which develop against the Fc portion of IgG. Usually, the antibodies are IgM, but occasionally can be IgG or rarely, IgA.

The IgM rheumatoid factor identified in routine laboratory testing has been found in 50-80% of patients with rheumatoid arthritis and is associated with more severe disease. There is some evidence that the presence of rheumatoid factors, and their interaction to form immune complexes, play a role in the pathogenesis of inflammation in rheumatoid arthritis. High titers also are associated with extraarticular involvement. With certain treatments, the titer may decrease, but the antibody usually does not disappear. In addition to rheumatoid arthritis, rheumatoid factors occur in a variety of other rheumatic diseases, including SLE, Sjogren’s syndrome, scleroderma and polymyositis. Also associated with RFs are chronic infectious diseases such as leprosy, tuberculosis and subacute bacterial endocarditis. The incidence of low titer positive RF tests rises with age, especially in women.

METHOD DESCRIPTION:

The classical tube dilution latex fixation procedure of Singer and Plotz is the method used in our lab to quantitate serum levels of rheumatoid factor. Sera are screened at a low and high dilution to prevent false negatives due to prozoning. The highest titer of patient serum at which agglutination is still visible is reported.

REFERENCE RANGE:

Negative. (Approximately 3% of the normal population have low titers, with incidence increasing with age.)

SPECIMEN REQUIREMENTS:

0.3 ml serum. Freeze.