Rheumatoid arthritis (RA) is the most prevalent systemic rheumatic disease, afflicting 1-2% of the population. Although the pathogenesis by the presence of various circulating autoantibodies, including rheumatoid factor (RF), anti-keratin antibodies (AKA) and anti-perinuclear factor (APF).

RF is present in 70-90% of patients with RA. The measurement of RF is therefore important in the diagnosis and prognosis of RA as high titres of RF occur in sera of patients who tend to develop extra-articular complications.

Agglutination is the most common method for measuring RF. Most agglutination methods detect 19S (pentameric) IgM RF only. ELISA methodology allows the measurement of all the major immunoglobulin classes.

The IgM RF shows a strong correlation with the onset of an erosive disease state. IgM RF is also present in patients with systemic lupus erythematosus (SLE). It has been suggested that 75% of patients with chronic polyarthritis have IgM RF, whereas, only 30% of patients with other connective tissue diseases have raised levels. IgM RF is also seen in other diseases such as viral hepatitis, liver cirrhosis, sarcoiditis and tuberculosis.

The IgG RF has been reported to be significantly raised in patients with rheumatoid vasculitis (a patchy inflammation of the walls of the small blood vessels) and correlate with disease activity. The IgG RF may also contribute to the tissue damage by activating complement.

The presence of IgA RF is indicative of the more severe and erosive outcome of RA. The detection of IgA RF can give an early indication of an underlying rheumatic disease and is considered to be more specific than the measurement of IgM RF.