Thursday, 11 November 2010

MRCP revision battle 46.6: Rheumatoid factor

Rheumatoid factor is an autoantibody against the Fc portion of IgG.  

Rheumatoid factor is found in 4% of the 'normal' population and 25% of elderly.

Rheumatoid factor is positive in low titres in chronic infections, for example:
  • 10% syphyllis
  • 20% pulmonary TB
  • 50% leprosy

Rheumatoid factor is often positive in high titres in connective tissue disease, for example it is positive in:
  • 70% of patients with RA
  • virtually 100% of patients with RA with extra-articular manifestations
  • >75% sjogrens
  • 20-40% SLE
  • 30% scleroderma

It may also be present in other immunological diseases, for example autoimmune liver disease or sarcoidosis.

Very high titres of rheumatoid factor may be found in cryoglobulinaemias

Rheumatoid factor is only useful in RA as an assessment of prognosis.  It is not useful for diagosis or or measuring level of activity.

Rheumatoid factor is detected by the Rose-Waaler test (sheep red cell agglutination) or the latex agglutination test.

Now to the final battle of the day - lichen planus