Rheumatoid factor

Rheumatoid factor or RF is an IgM autoantibody directed against the Fc portion of autologous IgG. It is present in 70% of cases of rheumatoid arthritis, and in many other autoimmune conditions.

It leads to the formation of immune complexes which activate complement with the formation of the strongly chemotactic factors C3a and C5a. It appears that, in rheumatoid arthritis, the antibody may be synthesised locally within the joints. Cell-mediated immunity may also be activated in RA. The presence of high levels of rheumatoid factor is associated with systemic complications such as vasculitis, neuropathy and nodules.

It is found in many normal members of the population, incidence increasing with age, in the absence of any pathology.

• can be positive in the normal population (c.5% prevalence, higher in the elderly), especially at low titre (e.g. <23 units/ml)
• will be positive in around 70% of patients with RA and >90% patients with primary Sjogren's syndrome
  • just as a normal ESR may be seen in RA, a negative rheumatoid factor does not rule RA out

Reference:

• 1) ARC (Autumn 2012). Hands On - The approach to the patient presenting with multiple joint pain; 7(1):1-12.