Rheumatoid Factor

The rheumatoid factor (RF) test is the most useful immunologic test for confirming rheumatoid arthritis (RA). In this disease, "renegade" immunoglobulin G (IgG) antibodies, produced by lymphocytes in the synovial joints, react with IgM antibody to produce immune complexes, complement activation, and tissue destruction. How IgG molecules become antigenic is still unknown, but they may be altered by aggregating with viruses or other antigens. Techniques for detecting RF include the sheep cell agglutination test and the latex fixation test. Although the presence of this autoantibody is diagnostically useful, it may not be etiologically related to RA.

Purpose

To confirm RA, especially when clinical diagnosis is doubtful.

Patient preparation

• Explain to the patient that this test helps confirm RA.
• Advise him that he needn't restrict food or fluids before the test.
• Tell him that the test requires a blood sample and who will perform the venipuncture and when.
• Reassure him that although he may experience transient discomfort from the needle puncture and the tourniquet, collecting the blood sample usually takes less than 3 minutes.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml red-top tube.
• Because a patient with RA may be immunologically compromised, keep the venipuncture site clean and dry for 24 hours.
• Check regularly for signs of infection.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Reference values

A normal RF titer is nonreactive if it's less than 39 IU/ml, weakly reactive if it's 40 to 79 IU/ml, and reactive if it's equal to or greater than 80 IU/ml.

Abnormal findings

Non-RA and RA populations aren't clearly separated with regard to the presence of RF: 25% of RA patients have a nonreactive titer, 8% of non-RA patients are reactive at greater than 39 IU/ml, and only 3% of non-RA patients are reactive at greater than 80 IU/ml.

Patients with various non-RA diseases characterized by chronic inflammation may test positive for RF. These diseases include systemic lupus erythematos, polymyositis, tuberculosis, infectious mononucleosis, syphilis, viral hepatic disease, and influenza.

Interfering factors

• Inadequately activated complement (possible false-positive)
• Serum with high lipid or cryoglobulin levels (possible false-positive, requiring repetition of the test after restricting fat intake)
• Serum with high IgG levels (possible false-negative due to competition with IgG on the surface of latex particles or sheep red blood cells used as substrate)