Rheumatoid factor (RF) is a type of rheumatoid arthritis. Normally antibodies are produced by the immune system to help destroy and eliminate invading bacteria and viruses that can cause disease. However, RF is an antibody that can attach to normal body tissue, resulting in damage.

High levels of rheumatoid factor can be caused by several autoimmune diseases (including rheumatoid arthritis) and some severe infections. Occasionally an elevated level of RF is present in healthy people.

The amount of rheumatoid factor in blood can be measured in two ways:

• Agglutination tests. One agglutination method mixes the blood being tested with tiny rubber (latex) beads that are covered with human antibodies. If rheumatoid factor (RF) is present, the latex beads clump (agglutinate). This method is best used as a first-time screening test for rheumatoid arthritis. Another agglutination method mixes the blood being tested with red blood cells of a sheep covered with rabbit antibodies. If RF is present, the red blood cells agglutinate. This method is often used to confirm the presence of RF.
• Nephlometry test. (“Nephlo” means cloud, and “metry” means measure.) This method mixes the blood being tested with antibodies that cause the blood to clump if rheumatoid factor (RF) is present. While a laser light passes through the tube containing the mixture, an instrument measures how much light is blocked by the sample in the tube. As levels of RF increase, more clumping occurs, causing a cloudier sample and less light to pass through the tube.

This test is done on a blood sample taken from a vein.

Rheumatoid Factor (RF)
Results

The results of the rheumatoid factor (RF) test may be reported in titers or units:

• A titer is a measure of how much the agglutination test blood sample can be diluted before RF can no longer be detected. A titer of 1 to 20 (1:20) means that RF can be detected when 1 part of the blood sample is diluted by up to 20 parts of a salt solution (saline). A larger second number means there is more RF in the blood. Therefore, a titer of 1 to 80 indicates more RF in the blood than a titer of 1 to 20.
• Nephlometry units indicate how much light is blocked by the blood sample in the tube. A high level of RF causes the sample to be cloudy, so less light passes through the tube than when the RF level is low. Therefore, an RF level of 100 units is higher than one of 40 units.

Normal

Normal values may vary from lab to lab.

Greater than normal values may mean

• A level of rheumatoid factor (RF) greater than a titer of 1 to 80 supports a diagnosis of rheumatoid arthritis, but it may also occur in other conditions.
• An RF titer between 1 to 20 and 1 to 80 may indicate conditions such as systemic lupus erythematosus (SLE), scleroderma, Sjögren's syndrome, tuberculosis, mononucleosis, infection of a heart valve (endocarditis), sarcoidosis, and some infections caused by a virus or parasite (such as malaria).