A 35-year-old man with a history of unprotected sexual contact 4 weeks ago presents with a painless genital ulcer. VDRL is reactive at 1:8 dilution. TPHA (Treponema pallidum haemagglutination assay) is also reactive. FTA-ABS is reactive. How should these results be interpreted and what is the significance of the TPHA remaining reactive lifelong?

• A VDRL is specific for treponema and confirms active syphilis; TPHA is a screening test that becomes negative after treatment
• B VDRL is a non-treponemal cardiolipin-based test that quantifies disease activity and becomes non-reactive post-treatment; TPHA/FTA-ABS are treponemal tests that detect antibodies to T. pallidum-specific antigens and remain reactive (serofast) lifelong even after successful treatment ✓
• C A single reactive VDRL at 1:8 is diagnostic of tertiary syphilis requiring LP
• D FTA-ABS positivity alone confirms active disease regardless of VDRL titre

Explanation

Syphilis serology uses two test types: (1) Non-treponemal tests (VDRL, RPR) detect IgG/IgM antibodies to cardiolipin-lecithin-cholesterol antigen (released from damaged host cells + treponemal lipoidal antigens). Titres correlate with disease activity; they decline and become non-reactive after successful treatment (titers fall 4-fold within 6–12 months). (2) Treponemal tests (TPHA, FTA-ABS, TPPA) detect antibodies specific to T. pallidum antigens — these confirm true treponemal infection but REMAIN REACTIVE LIFELONG even after cure ('serofast'), so they cannot be used alone to assess treatment response. The clinical scenario (painless ulcer + all tests reactive) represents primary syphilis; treatment is benzathine penicillin G 2.4 MU IM single dose.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.