A sex worker presents with a painless clean-based genital ulcer with indurated margins and a firm rubbery inguinal lymph node. Dark-field microscopy confirms spirochetes. Which serological test becomes positive EARLIEST in primary syphilis?

• A VDRL
• B TPHA (Treponema pallidum hemagglutination assay)
• C FTA-ABS (fluorescent treponemal antibody absorbed) ✓
• D RPR (rapid plasma reagin)

Explanation

FTA-ABS is the first serological test to become positive in primary syphilis, turning positive within 2-3 weeks of infection, even before the chancre appears. Nontreponemal tests (VDRL, RPR) become positive approximately 4-6 weeks post-infection and may be negative in early primary syphilis. TPHA becomes positive after FTA-ABS. Once FTA-ABS becomes positive, it remains positive lifelong regardless of treatment. This is why FTA-ABS (or TPPA) is used for confirmatory testing.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.