Rheumatoid factor
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Background
- Rheumatoid factors (RFs) are autoantibodies directed against the Fc portion of IgG.
- Most commonly IgM, but can also be IgG or IgA.
Mechanism
- RF binds to IgG, forming immune complexes.
- These deposit in joints and tissues, activating complement and driving inflammation in rheumatoid arthritis (RA).
- Not a direct inflammatory marker (like CRP/ESR), but part of the autoimmune process.
Clinical use
Diagnostic marker:
- Positive in ~70–80% of patients with RA (but not specific).
- Sjögren’s syndrome
- SLE
- Chronic infections (e.g. hepatitis C, TB, subacute bacterial endocarditis)
- Sometimes in healthy elderly individuals
Prognostic marker:
- High titres of RF are associated with more severe disease, extra-articular manifestations, and worse outcomes in RA.
RF is not for disease activity monitoring, ESR/CRP are used instead.
Testing
- Usually detected by nephelometry, ELISA, or latex agglutination assays.
- Reported as positive/negative or in titre levels.
Related autoantibodies
Anti-CCP (anti-cyclic citrullinated peptide antibodies):
- More specific for RA (~95%).
- Can appear earlier than RF.
- Often used alongside RF for diagnosis and prognosis.
Key Points
- RF is positive in approximately 70% of individuals with Rheumatoid arthritis and 5% of the unaffected population.
- RF is an antibody (IgM) directed against IgG (Fc Portion)
- High titres are associated with severe RA but RF not used to monitor inflammation
- Anti-CCP (cyclic citrullinated peptide) is preferred as a test for rheumatoid arthritis as it is present in 90-95% of the rheumatoid population