What is the Rheumatoid Factor?
The rheumatoid factor is an antibody against IgG. These immune complexes contribute to rheumatologic diseases.
While it is found in 80 percent of adults with Rheumatoid Arthritis, it is very uncommon in children. It is positive in only approximately 10 percent of children with Juvenile Rheumatoid Arthritis (JRA) and only in the Polyarticular JRA subset.
When Should it Be Tested?
We recommend checking it only in patients with polyarticular, chronic arthritis, as it helps determine prognosis. A positive rheumatoid factor is associated with more persistent, erosive arthritis and can also be associated with extra-articular manifestations such as rheumatoid nodules.
Due to its low sensitivity, we do not recommend using the rheumatoid factor as a screening test for JRA. In addition, the rheumatoid factor is not entirely specific for JRA and therefore can yield false positive results, particularly in infections such as endocarditis and other autoimmune diseases including lupus.
There is a newer test, an anti-cyclic citrullinated peptide (CCP), which also has low sensitivity but has a high specificity; therefore, when this antibody is positive, it is very suggestive of Polyarticular JRA and is associated with more chronic, erosive disease.