A 56-year-old retired bus driver presents with symmetrical deforming arthropathy. His rheumatoid factor is positive at a very high titer. Regarding rheumatoid factor; all of the following are true, except?

a- Is an antibody directed against Fc portion of IgG.
b- May be of IgG, IgM, or IgA class.
c- Is diagnostic of rheumatoid arthritis.
d- Can be detected by many lab methods.
e- Found in almost 100% of cases of secondary Sjogren's syndrome and Felty’s syndrome.

The correct answer is c.

• a - true, is an autoantibody.
• b - but, we detect only IgM in the slide agglutination test.
• c - false; does not reflect any thing apart from poor prognosis if found in high titer in RA.
• d - like slide tube agglutination, SCAT, and Rose Waaler tests. e-true; these are seropositive.

In terms of sensitivity, it is present in the majority of patients with erosive rheumatoid disease but may only appear after months or years of disease, once the diagnosis is beyond dispute. It is therefore neither sufficient nor necessary for the diagnosis. Its principal use is as a prognostic marker; a high titer at the onset of rheumatoid arthritis associates with a poorer prognosis. IgG rheumatoid factor has greater specificity for major rheumatic disease but the above caveats still remain.