Rheumatoid Factor (RF) Blood Test

What is rheumatoid factor?

Rheumatoid factor (RF) is an autoantibody: an antibody that mistakenly attacks the body's own proteins, specifically the Fc region of IgG immunoglobulin. It is produced by the immune system in rheumatoid arthritis (RA) and several other conditions. The RF test is one of the oldest and most widely used blood tests in rheumatology, though it has important limitations in specificity. It is almost always ordered alongside anti-CCP antibodies, CRP and ESR when evaluating joint pain.

Rheumatoid factor normal range

What does a POSITIVE RF mean?

A positive RF in the setting of symmetrical small joint pain, morning stiffness lasting >1 hour, and swollen joints strongly supports rheumatoid arthritis. About 70-80% of people with RA are RF-positive (seropositive RA). RF-positive RA tends to be more severe, with greater joint destruction and systemic complications than seronegative RA. However, RF is not diagnostic alone, about 5% of healthy people are RF-positive, and the rate increases with age.

Other causes of positive RF

RF vs Anti-CCP: which is better?

Anti-CCP antibodies (anti-cyclic citrullinated peptide) are more specific for rheumatoid arthritis than RF. Anti-CCP is positive in about 95% of RA cases but rarely positive without RA. Anti-CCP also appears earlier in RA, sometimes years before symptoms, and is not raised by hepatitis C or other conditions that falsely raise RF. If both RF and anti-CCP are positive, the diagnosis of RA is highly likely. If only RF is positive, other causes should be considered.

Questions to ask your doctor

• Should I also test anti-CCP antibodies?
• Do I need to see a rheumatologist?
• How does my RF level correlate with disease severity?
• Are CRP and ESR also elevated?
• Do I need X-rays of my hands and feet?

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Ingegnoli F, Castelli R, Gualtierotti R. Rheumatoid factors: clinical applications. Dis Markers. 2013;35(6):727-34. doi:10.1155/2013/726598 · PMID 24324289
2. Rheumatoid Factor. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK532898

References

Sources cited on this page. PubMed links open the original abstract.

1. Nishimura K, Sugiyama D, Kogata Y, et al. Meta-analysis: diagnostic accuracy of anti-cyclic citrullinated peptide antibody and rheumatoid factor for rheumatoid arthritis. Ann Intern Med. 2007;146(11):797–808. PMID 17609319 · doi:10.7326/0003-4819-146-11-200706050-00008

What rheumatoid factor actually detects

Rheumatoid factor (RF) is an antibody – usually IgM, though also IgA and IgG subtypes exist – directed against the Fc (constant) region of IgG antibodies. In essence, it is an antibody against another antibody. In rheumatoid arthritis, abnormal immune activation leads to the production of RF by B cells in the synovial membrane of inflamed joints, from where it enters the bloodstream.

RF is measured either as a titre (above a certain dilution being positive) or as a quantitative level in IU/mL. Most labs report it as positive or negative with a quantitative value. A high-positive RF (3× the upper limit of normal or higher) is more specific for RA than a low-positive result.

Why RF alone does not diagnose RA

RF has both sensitivity and specificity limitations that make it insufficient as a standalone diagnostic test:

• False negatives: Approximately 20–30% of RA patients are RF-negative ("seronegative RA"). These patients can have equally severe disease. The combination of RF and anti-CCP together maximises detection – seronegative for both ("double seronegative RA") is a specific subtype, sometimes representing a different disease process.
• False positives: RF is elevated in many other conditions:
  • Hepatitis B and C (immune complex formation)
  • Sjögren's syndrome (very high titres – often higher than in RA)
  • Systemic lupus erythematosus
  • Infective endocarditis
  • Sarcoidosis
  • Cryoglobulinaemia
  • 5–10% of healthy older adults (RF prevalence increases with age)

A positive RF in a person without joint symptoms should not be labelled as "rheumatoid arthritis" – it requires clinical correlation with symptoms, physical examination, and anti-CCP testing.

RF titres and disease severity

High-positive RF (above 3× ULN) – particularly when combined with positive anti-CCP – is associated with a more aggressive RA phenotype: faster radiographic progression, greater joint destruction, higher rates of extra-articular manifestations (rheumatoid nodules, vasculitis, interstitial lung disease, Felty's syndrome). This high-risk serology profile supports earlier and more aggressive DMARD therapy and closer monitoring for pulmonary complications.