Rheumatoid factor

Upper limits of 14 to 20 IU/mL are printed depending on the method; some laboratories report only Negative or Positive, or a titre. About 1 in 20 healthy adults are positive at a low level in the general population, rising with age.

Rheumatoid factor is an antibody that is directed against the body's own antibodies, binding to the tail end of IgG. It is found in most people with rheumatoid arthritis, which is how it got its name, but it also appears in a number of other conditions and in a fair proportion of healthy people, particularly as they get older.

Doctors request it when looking at joint pain and swelling, usually alongside anti-CCP antibodies, CRP and ESR. It is one of the tests used when looking at some other autoimmune conditions and at long-term infections.

• Rheumatoid arthritis, in about three quarters of people with it
• Sjogren's syndrome and some other autoimmune conditions
• Long-term infections, including hepatitis C and tuberculosis
• Older age, in the general population
• Cryoglobulins and some blood conditions
• Some lung and liver conditions

• Treatment of rheumatoid arthritis lowers it in some people
• A negative result is the usual finding in healthy adults; there is no lower limit
• Around a quarter of people with rheumatoid arthritis never have a detectable level

• About 1 in 20 healthy adults, and more among older people, have a low positive level with no joint disease
• A recent viral infection or vaccination can produce a temporary positive
• Methods differ between laboratories, so a number from one lab does not map onto another's range

No fasting needed. A single value is read alongside anti-CCP, CRP and ESR; the number is not usually followed over time.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.