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Syphilis Serology — SCE Infectious Diseases MCQ

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HardSTIsSyphilis SerologySCE Infectious Diseases

A 30-year-old man with HIV (CD4 250, VL <50 on ART) is found to have a positive syphilis serology at routine screen. RPR is 1:2 and TPHA is positive. He had a documented RPR of 1:64 and was treated for secondary syphilis 12 months ago with Benzathine Penicillin G 2.4 MU IM. What does this serological pattern indicate?

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Correct answer: C — Adequate serological response to treatment (fourfold decline from 1:64 to 1:2); the persistent low-titre RPR is a serofast state

A fourfold (two-dilution) decline in RPR titre within 6–12 months of treatment (from 1:64 to ≤1:16 and now 1:2) indicates an adequate serological response. Many patients, particularly those with HIV, may not achieve complete RPR seroreversion (negative RPR) and instead become 'serofast' — persistently low-titre positive. A serofast state does NOT require retreatment. Treatment failure is defined as failure to achieve a fourfold decline or a fourfold rise in titre.

Reference: BASHH 2019 – Syphilis guidelines; CDC 2021 – STI treatment