A 28-year-old man presents with a painless ulcer on the glans penis for 3 weeks. On examination, the ulcer has a clean base with indurated edges and regional lymphadenopathy. He denies systemic symptoms. Which investigation is most appropriate to confirm the diagnosis of primary syphilis?

## Investigation of Choice for Primary Syphilis ### Clinical Context The patient presents with a classic **painless indurated ulcer (chancre)** with regional lymphadenopathy — hallmark of primary syphilis. The diagnosis must be confirmed by direct visualization of the causative organism, *Treponema pallidum*. ### Why Dark-Field Microscopy is the Gold Standard **Key Point:** Dark-field microscopy (DFM) is the **most specific and earliest confirmatory test** for primary syphilis because it directly visualizes motile spirochetes in fresh exudate from the ulcer. **High-Yield:** - DFM can detect *T. pallidum* within **24–48 hours** of symptom onset - Sensitivity is **75–95%** when performed on fresh exudate - Requires fresh, warm specimen (organisms are motile and die quickly) - Does not require serological conversion — positive even in early primary syphilis when antibodies are absent ### Diagnostic Timeline in Syphilis | Stage | Clinical Feature | DFM | RPR/VDRL | FTA-ABS/TP-PA | |-------|------------------|-----|----------|---------------| | **Primary** | Chancre | ✓ Positive (earliest) | May be negative initially | May be negative initially | | **Secondary** | Rash, lymphadenopathy | Positive in lesions | ✓ Highly positive | ✓ Positive | | **Latent** | Asymptomatic | Negative | Positive | Positive | | **Tertiary** | Gummas, neurosyphilis | Negative | Variable | Positive | **Clinical Pearl:** In primary syphilis, the patient may still be **seronegative** (RPR/VDRL negative) because antibodies take 1–4 weeks to develop. Therefore, serological tests alone cannot rule out primary syphilis — direct visualization is essential. ### Limitations of Other Tests **Rapid Plasma Reagin (RPR) & VDRL:** - Non-treponemal tests (detect IgM/IgG against cardiolipin) - May be **negative in early primary syphilis** (before antibody production) - Positive in secondary, latent, and tertiary syphilis - Not suitable for diagnosis of primary syphilis when seronegative **FTA-ABS & TP-PA:** - Treponemal-specific tests - Also may be negative in very early primary syphilis - Become positive later in primary stage - Used for **confirmation** after serology, not initial diagnosis ### Practical Approach ```mermaid flowchart TD A[Suspected Primary Syphilis<br/>Painless Indurated Ulcer]:::outcome --> B{Perform Dark-Field<br/>Microscopy}:::decision B -->|Positive| C[Confirm Diagnosis<br/>Start Treatment]:::action B -->|Negative| D[Repeat DFM or<br/>Perform Serology]:::action D --> E[RPR/VDRL + FTA-ABS]:::action E --> F{Serology Positive?}:::decision F -->|Yes| G[Diagnose Syphilis<br/>Treat]:::action F -->|No| H[Consider Other Causes<br/>of Genital Ulcer]:::outcome ``` **Mnemonic:** **DFM = Direct, Fast, Motile** — Dark-field shows the motile spirochete directly in primary syphilis.