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Rheumatoid Factor Definition

Rheumatoid factor (RF) is an antibody produced by the body's immune system. Like other antibodies, it is a protein, but an unusual one in that it can attack healthy tissue in the body. RF is also the name given to the test that measures the amount of this antibody in the blood, which in healthy individuals should be relatively low. High levels of RF are usually---but not always---a sign that some form of autoimmune disease is present.

  1. More about RF

    • Because it is an antibody that can attack healthy tissue, RF is considered an autoantibody. Like other antibodies produced by the immune system, it is an immunoglobulin. However, it is unique in its ability to bind to other antibodies, which normally play a key role in fighting off infections that invade the body.

    Autoimmune Diseases

    • Elevated levels of RF in the blood are used to help diagnose such autoimmune disorders as rheumatoid arthritis, Sjogren's syndrome, adult-onset Still's disease, dermatomyositis, sarcoidosis, scleroderma and systemic lupus erythematosus. The RF test generally is used in conjunction with a physical exam and other blood tests, according to MayoClinic.com. Such blood tests may include ANA (antinuclear antibody), anti-CCP (anti-cyclic citrullinated peptide antibody), CBC (complete blood count), CRP (C-reactive protein), and ESR (erythrocyte sedimentation rate).

    Other Reasons for High RF

    • MedlinePlus indicates that elevated RF levels may be present in people with diseases that are not autoimmune in character. The RF test, however, is not used in the diagnosis of these diseases, which include viral infections, such as AIDS, hepatitis and influenza; bacterial infections, such as endocarditis and tuberculosis; parasitic infections; cancer; and chronic liver and lung diseases. For reasons not completely understood, the RF levels of some completely healthy people may be elevated as well.

    RF Test

    • The test for RF is performed in the same way as other blood tests. The inside of your arm---or an alternate site for blood withdrawal---will be wiped with antiseptic. The phlebotomist then will withdraw a vial of blood for RF analysis; additional vials of blood may be taken if multiple tests have been ordered. The blood is then sent to a laboratory for analysis. The results will be returned to your doctor who, in turn, should notify you.

    Test Results

    • A normal level of RF in an adult is 30 u/mL (units per milliliter) or less, according to Southwest Medical Center's Normal Reference Range Table. Negative RF test results indicate that the amount of RF in the blood is very low, according to MayoClinic.com. This would not completely rule out the possibility of autoimmune disease, because some people with such diseases may have low RF levels. By the same token, a positive RF test indicates a high level of RF in the blood, but is not necessarily conclusive proof that an autoimmune disorder is present, because some healthy individuals have elevated RF but no disease.

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