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Low-Titre RF Significance — SCE Rheumatology MCQ

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EasyAutoimmune SerologyLow-Titre RF SignificanceSCE Rheumatology

A 55-year-old woman has symmetrical clinical synovitis of the metacarpophalangeal and proximal interphalangeal joints for 8 weeks. Rheumatoid factor is 28 IU/mL (laboratory upper limit of normal 20 IU/mL), while anti-CCP antibodies are negative. Which interpretation of these serological findings is most accurate?

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Correct answer: C — The low-positive rheumatoid factor modestly supports rheumatoid arthritis but is nonspecific; negative anti-CCP reduces its probability without excluding it.

This RF result is low-positive because it is above the assay upper limit of normal but below three times that limit. Low-positive RF provides some support for RA but has limited specificity: RF can occur in other autoimmune diseases, chronic infections and healthy individuals. Anti-CCP is more specific for RA, so a negative result lowers the diagnostic probability but does not exclude RA. The persistent symmetrical small-joint synovitis remains clinically important and warrants assessment for RA regardless of serology. Therefore A and E overinterpret RF, C incorrectly treats low-positive RF as having no value, and D incorrectly uses anti-CCP as an exclusion test. Strictly, this patient should not be described as having “seronegative RA” because RF is positive.

Reference: NICE, Rheumatoid arthritis in adults: management (NG100), section 1.1 Referral, diagnosis and investigations, updated 2020: https://www.nice.org.uk/guidance/ng100/chapter/Recommendations