A VDRL positive result at 1:16 is found incidentally in a 28-year-old man with no symptoms. TPHA is positive. He claims he completed penicillin treatment for syphilis 6 months ago. What serological response confirms adequate treatment?

• A VDRL becoming non-reactive within 3 months of treatment confirms cure; if reactive at 6 months, re-treatment is mandatory
• B FTA-ABS IgM becoming negative confirms cure; FTA-ABS IgG remains positive indefinitely
• C Fourfold (two-dilution) decline in VDRL titre (e.g., 1:16 → 1:4) within 6–12 months for primary/secondary; TPHA may remain positive for life (serofast) regardless of cure ✓
• D TPHA titre decreasing fourfold is the marker for treatment adequacy; VDRL titre has no role in follow-up

Explanation

Non-treponemal tests (VDRL, RPR) quantify anticardiolipin (reagin) antibodies that decline with successful treatment — a fourfold (two-dilution) decrease in titre (e.g., 1:16 → 1:4, or 1:32 → 1:8) within 6–12 months after treatment indicates adequate response in primary/secondary syphilis. Treatment failure or re-infection is suspected if titre fails to decline fourfold or rises fourfold. Treponemal tests (TPHA, FTA-ABS, TPPA) once positive usually remain positive for life regardless of treatment ('serofast') and cannot be used to monitor treatment response. VDRL may become non-reactive eventually in some treated patients but this is not a required endpoint.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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