Rheumatoid Factor (RF)

Normally, your immune system makes proteins called antibodies to attack germs that could make you sick. Rheumatoid factor, however, does not attack germs. It attacks healthy cells, usually immunoglobulin G (IgG), one of the most common antibodies made to fight germs. Your immune system makes rheumatoid factor by mistake or when it's overactive fighting an infection.

Many people have small amounts of RF in their blood and are healthy. Even high amounts aren't always a sign of a health condition. But if you have certain symptoms and higher levels of rheumatoid factors, you may have an autoimmune disorder or another health condition related to high RF levels.

The results of an RF test alone cannot diagnose a condition. However, your provider can use these results, along with your medical history and the results of other tests, to determine if you have rheumatoid arthritis or any other condition that causes high RF levels. Generally, your results will be either positive or negative.

A negative (normal) result means that you have little or no rheumatoid factor in your blood. But that doesn't rule out rheumatoid arthritis or another health problem. If you have symptoms of rheumatoid arthritis, but your RF test results are normal, your provider may order more tests to make a diagnosis. About 25% of people with rheumatoid arthritis test negative for RF.

A positive (abnormal) result means that a higher level of rheumatoid factor was found in your blood. Besides rheumatoid arthritis, elevated RF can be seen with other connective-tissue conditions such as lupus, scleroderma, and Sjögren's syndrome, and with chronic infections like tuberculosis, leprosy, and syphilis. It can also occur with thyroid disease, some cancers, and in some otherwise healthy older adults. Your provider will also consider other test results along with your symptoms and medical history to find out the cause.

In rheumatoid arthritis, the size of the RF level doesn't track day-to-day disease activity, but very high levels are linked to a higher chance of more severe joint disease. Providers often order an anti-CCP antibody test together with RF, because anti-CCP is more specific for rheumatoid arthritis and the two together help confirm the diagnosis.

Sometimes, rather than a negative or positive result, your results may tell you how much RF was in the sample. Your results may also give a normal range for RF for that laboratory.