Rheumatoid Factor

Rheumatoid factor is an antibody against the Fc portion of IgG (also an antibody). They can belong to any isotype of immunoglobulin e.g. IgM, IgG and IgE - and any of these can be detected in the blood test.¹

Any patient who may have arthritis. But keep in mind it may be negative in rheumatoid disease. Many are seronegative to begin with but 80% seroconvert (become positive).² Disease severity is often worse in those who are seropositive.³

Venous blood taken in clotted tube - usually same one as U + E.

Results can be reported in titres (normal < 1:20) but more commonly as units and <23u is normal (normal levels may vary from laboratory to laboratory). The sensitivity and specificity of rheumatoid factor for rheumatoid disease is low and thus it is not a good screening test. However, the predictive value of rheumatoid factor in patients with symmetric polyarticular joint swelling is 80%.

1. Significant associations - rheumatoid disease, Felty's syndrome. The higher the level in rheumatoid disease the more worse the joint destruction and greater chance of systemic involvement.
2. Insignificant associations - any inflammatory condition e.g. Sjogrens disease, systemic lupus erythematous and mixed connective tissue disorder (see table).

Rheumatoid factor does not generally help in monitoring disease - although it may help with the use of the newer agents etanercept and infliximab. In patients on etanercept or infliximab and disease modifying antirheumatic drugs the levels of rheumatoid factor reduces which is associated with reduced clinical disease activity.^5,6

It can predict disease outcome in some patients¹, one example of this is that radiological progression i.e. changes in hand x rays are worse in those who are seropositive.⁷