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Procalcitonin in SLE — SCE Rheumatology MCQ

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HardSLE & Antiphospholipid SyndromeProcalcitonin in SLESCE Rheumatology

A 55-year-old woman with systemic lupus erythematosus presents with fever. Her CRP is 85 mg/L and procalcitonin is 0.8 ng/mL. Anti-dsDNA titres are rising and C3 and C4 concentrations are falling. Which interpretation of the procalcitonin result is most appropriate?

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Correct answer: E — The result increases suspicion of bacterial infection, but PCT has imperfect sensitivity and requires clinical and microbiological correlation.

Explanation lettering: E = shown as A · A = shown as B · B = shown as C · C = shown as E

C is correct. In febrile SLE, PCT is, on average, higher with bacterial infection than with non-infectious disease activity and may provide useful rule-in information. However, pooled sensitivity and specificity are only moderate and reported thresholds are heterogeneous; therefore, a PCT of 0.8 ng/mL raises concern but is not diagnostic. Rising anti-dsDNA and falling complement support active lupus but do not exclude concurrent infection. A and E incorrectly dismiss potentially useful adjunctive information. B treats an imperfect biomarker as a rule-out test, risking inappropriate immunosuppression. D is unsafe because infection and flare may coexist. PCT should be interpreted alongside clinical assessment, cultures, imaging, CRP and serial results rather than used as a standalone discriminator.

Reference: Bruera S, Ventura MJ, Agarwal SK, et al. The utility of erythrocyte sedimentation rate, C-reactive protein, and procalcitonin in detecting infections in patients with systemic lupus erythematosus: a systematic review. Lupus. 2022;31:1163-1174. https://pubmed.ncbi.nlm.nih.gov/35650026/