A patient with a painless genital ulcer has negative dark-field microscopy and VDRL is non-reactive on day 5 of the ulcer. The MOST appropriate next diagnostic step is:
- A Repeat VDRL in 2–4 weeks and perform treponemal NAAT (PCR) on ulcer swab ✓
- B Start benzathine penicillin empirically — serology will never be positive in primary syphilis
- C Diagnose chancroid and treat with azithromycin 1 g stat
- D Perform TPHA immediately — it is always reactive in primary syphilis
Explanation
In early primary syphilis, VDRL/RPR may be non-reactive in up to 30% of patients in the first 1–2 weeks as antibodies are still rising. Dark-field microscopy is highly specific but requires expertise and living organisms. Treponemal PCR on ulcer swab has excellent sensitivity (>95%) in primary syphilis and can be positive before serology. Repeating serology in 2–4 weeks allows seroconversion to be detected. TPHA becomes reactive slightly earlier than VDRL but is also not universally positive in very early primary syphilis. Empirical treatment is appropriate in high-risk contacts but diagnosis should be pursued.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.