Rheumatoid factor

What is being tested?
This test detects rheumatoid factor (RF), a particular type of autoantibody. An antibody is a protein circulating in the blood that is typically made in response to the presence of material foreign to the immune system, usually another protein, called an antigen.

Autoantibodies are antibodies directed against the person’s own (“self”) proteins rather than against foreign agents, such as bacterial proteins. Rheumatoid factor is an autoantibody directed against fragments of the IgG class of immunoglobulins, a class that is usually raised in inflammatory conditions. Rheumatoid factor rises in most patients with inflammation, which is why it is a sensitive test for assessing the degree of inflammation associated with rheumatoid arthritis. Because of its low specificity, however, it is not a diagnostic test. Even experts do not fully understand how and why it forms, and although it has no direct role in joint damage, it seems to take part in the inflammatory reaction and contribute to the tissue destruction typical of rheumatoid arthritis

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
The rheumatoid factor (RF) test is used in the diagnosis of rheumatoid arthritis (RA). It can also be used in the diagnosis of Sjögren’s syndrome, a condition closely related to arthritis. About 80-90% of patients with this condition have high RF values.

When is it requested?
The test is requested when there are symptoms typical of RA. Symptoms can include prolonged morning joint stiffness, weight loss, nodules under the skin and X-ray evidence of swollen joint capsules with loss of cartilage and bone, if the disease is advanced.

If there are symptoms typical of RA but the first RF test is negative, the test should be repeated. Values change with the degree of symptoms and inflammation present, and may be low when the disease is in remission or inactive.

The test can be useful in diagnosing Sjögren’s syndrome. Symptoms include a dry mouth and eyes, dry skin, and joint and muscle pain. Most connective tissue disorders are autoimmune diseases, and patients with Sjögren’s syndrome often have other conditions too, such as Raynaud’s syndrome, scleroderma, autoimmune thyroid disorders and systemic lupus erythematosus.

What does the test result mean?
The presence of RF indicates possible rheumatoid arthritis (RA). A positive test has been found in most cases of RA. In addition, more than 50% of patients with high RF levels have Sjögren’s syndrome. Many patients with RA also have Sjögren’s syndrome (women very often have both disorders). Women are two to three times more likely than men to have RA, and in 90% of cases there is also Sjögren’s syndrome

If the RF test is positive but both RA and Sjögren’s syndrome have been ruled out, there may be other reasons for the positive result, such as endocarditis, systemic lupus erythematosus (SLE), tuberculosis, syphilis, sarcoidosis, cancer, viral infections, or liver, lung or kidney disease. A positive result can also be found after a kidney or skin transplant from a donor with a different genetic profile.

A negative result may mean that there is no RA, that the test was done too early in the course of the disease, or that the patient is in remission. If there are suggestive and typical symptoms, the doctor may have the RF test repeated.

Is there anything else I should know?
The test has a high rate of false positives, and to diagnose RA or Sjögren’s syndrome it must be used together with the patient’s symptoms and medical history.

Factors that interfere with the test include vaccinations, lipaemia (large amounts of fat in the blood) and incorrect handling of samples. Methyldopa, a medicine used for high blood pressure, can also raise RF levels