A patient with early secondary syphilis has a positive VDRL at 1:128 and a positive TPPA. After treatment with benzathine penicillin 2.4 MU IM, VDRL is repeated at 6 months and shows 1:16. How should this serological response be interpreted?
- A Serological failure — re-treatment is required as VDRL should be non-reactive by 6 months
- B Serofast reaction — no further action is needed as TPPA will always remain positive
- C Adequate response — a ≥ 4-fold (2-dilution) decline in titer indicates successful treatment ✓
- D The patient has developed a prozone phenomenon
Explanation
A four-fold (two-dilution) decline in VDRL/RPR titer is the accepted criterion for adequate serological response to syphilis treatment. From 1:128 to 1:16 represents a 3-dilution (8-fold) decline, which satisfies the criterion for treatment success. VDRL/RPR may persist at low titers for months to years ('serofast') after adequate treatment, especially in late or secondary syphilis — this is not failure. TPPA (treponemal test) remains reactive for life regardless of treatment and is not used to assess treatment response. Prozone occurs with very high antibody concentrations causing false-negative results in undiluted serum.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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