A patient's syphilis serology shows: VDRL reactive 1:32, TPHA reactive. After treatment with benzathine penicillin G 2.4 MU IM, retesting at 6 months shows VDRL 1:4, TPHA reactive. How should this result be interpreted?

• A Treatment failure — TPHA remains reactive indicating ongoing infection
• B Adequate serological response — ≥4-fold VDRL titre fall indicates treatment success ✓
• C Biological false positive — TPHA titres should also fall post-treatment
• D Reinfection — VDRL titre must return to non-reactive to confirm cure

Explanation

A ≥4-fold (two dilution) fall in non-treponemal titre (VDRL/RPR) within 6–12 months of treatment is the accepted criterion for adequate serological response. TPHA (a treponemal test) typically remains reactive for life after successful treatment and should NOT be used to monitor treatment response. VDRL falling from 1:32 to 1:4 represents a 4-fold decrease (two dilutions), confirming adequate response. VDRL need not become non-reactive to indicate success, especially in late-stage or recurrent infection where 'serofast' reactions are common.

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

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