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Anti-Sm Antibodies — SCE Rheumatology MCQ

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ModerateSLE & Antiphospholipid SyndromeAnti-Sm AntibodiesSCE Rheumatology

A 32-year-old woman is being evaluated for suspected systemic lupus erythematosus (SLE). Her antinuclear antibody test is positive, and extractable nuclear antigen testing detects anti-Smith (anti-Sm) antibodies. Which statement best describes the significance of this result?

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Correct answer: D — It is highly specific for SLE but has low sensitivity

Anti-Sm is highly specific for SLE, so a positive result provides strong diagnostic support and anti-Sm is included among the SLE-specific antibodies in the 2019 EULAR/ACR classification criteria. Sensitivity is low, however, so a negative anti-Sm result does not exclude SLE. Exact specificity and sensitivity estimates vary with the assay and positivity threshold, making an invariant claim of greater than 99% specificity too absolute. Drug-induced lupus is classically associated with anti-histone antibodies. Anti-Ro/SSA and anti-La/SSB are more characteristic of Sjögren disease. Anti-Sm positivity may be associated with particular phenotypes in cohort studies, but its titre is not a reliable isolated marker of renal activity; anti-dsDNA levels, complement and clinical renal assessment are more useful.

Reference: van Beers JJBC, Schreurs MWJ. Anti-Sm antibodies in the classification criteria of systemic lupus erythematosus. Journal of Translational Autoimmunity. 2022;5:100155. https://pubmed.ncbi.nlm.nih.gov/35464346/