Some background about Rheumatoid Factor (RF):

RF is an antibody, usually of the IgM class, which usually binds to the Fc region of IgG. Because of that it is called an autoantibody (it recognises a normal body protein, IgG in this case). In the case of RA the resultant immune complexes contribute to joint damage.

Elevated RF levels can occur in people with other auto-immune diseases (eg, SLE and Sjögren's Syndrome). Elevated RF levels can also occur in many other diseases which do not have an auto-immune component; leprosy, syphilis, malaria, leukaemia, hepatitis and cirrhosis among many others.

High levels of RF are present in 80% of people with RA and 70% of people with Sjögren's Syndrome.

The clinical significance (if any) of elevated RF levels in the other diseases is not clear