My Rheumatologist Ordered A Rheumatoid Factor- What Is That?

There are four main antibodies in the blood. They are IgG, IgM, IgA, and IgE. An antibody is a protein made by white blood cells. The purpose of an antibody is to fight off infections and to destroy any other invaders that might do harm. For example, say, you get a splinter. Your white blood cells rush to that area and mount an acute inflammatory response. Antibodies are produced to attack bacteria. The area around the splinter becomes swollen, red, and hurts. Your immune system is doing its job.

Sometimes, though, a person can develop an autoimmune disease, meaning a disease where antibodies are created that are abnormal and are directed against the bodies own tissues. This creates a situation where there is chronic ongoing inflammation. The inflammation doesn't shut off. This inflammation eventually causes damage to the body.

Rheumatoid arthritis (RA) is an autoimmune disease. One of the first abnormalities in RA is the creation of rheumatoid factor. The rheumatoid factor (RF) is an antibody directed against another antibody called IgG. Most often the rheumatoid factor antibody is an IgM antibody. Sometimes it can be an IgA or an IgG. The level of rheumatoid factor can be measured using a specific blood test.

The RF is not diagnostic for rheumatoid arthritis though. Patients with early RA can be negative for rheumatoid factor. Between 10 and 20% of patients will be persistently negative for rheumatoid factor throughout their illness. However, between 80-90% of patients with RA will be positive for RF at some time during the course of their disease. While RF by itself is not diagnostic of RA, it is one of many criteria used to help with making the diagnosis.

The level of RF is also a good prognostic indicator since high levels of RF are associated with increased disease severity, the development of erosions (damage to the joint), involvement of other organ systems, and disability.

RF is not specific for RA and can be found in patients with other diseases such as systemic lupus erythematosus, spondyloarthropathy, inflammatory muscle disease, viral infections, vasculitis, reactive arthritis, mixed cryoglobulinemia, sarcoidosis, bacterial endocarditis, syphilis, and leprosy. RF can also be present in older people who have no other illnesses.

A more specific test for rheumatoid arthritis that is often ordered along with the RF is the anti-cyclic citrullinated peptide antibody (anti-CCP). The anti-CCP is probably less sensitive than the RF so both tests probably should be ordered at the same time. The anti-CCP can be negative in patients with RA, so it's important to look at the big picture rather than focus on the results of one test.

Finally, since RF may be IgG, IgM, or IgA, it's important to look for all three of these RFs. A note of caution...most laboratories only look for IgM rheumatoid factor. Make sure your rheumatologist looks for all three.