## Investigation of Choice for Primary Syphilis ### Dark-Field Microscopy: Gold Standard for Early Diagnosis **Key Point:** Dark-field microscopy of exudate from an active primary chancre is the MOST SPECIFIC and EARLIEST confirmatory test for syphilis. It directly visualizes motile Treponema pallidum organisms. **High-Yield:** Dark-field microscopy is: - Positive in primary syphilis (>95% sensitivity when performed correctly) - Positive in secondary syphilis from moist lesions - Becomes negative as the lesion ages or dries - Requires fresh exudate and immediate examination - Shows characteristic corkscrew motility of spirochetes ### Why Dark-Field Microscopy Is Superior in Primary Syphilis 1. **Timing:** Antibodies (non-treponemal and treponemal) may not yet be detectable in very early primary syphilis (first 1–2 weeks) 2. **Directness:** Identifies the organism itself, not antibodies 3. **Immediate Result:** Provides rapid diagnosis at the bedside ### Comparison of Serological Tests in Primary Syphilis | Test | Sensitivity in Primary | Specificity | Titer Rises | Use | |------|------------------------|-------------|-------------|-----| | VDRL/RPR (non-treponemal) | 70–80% | 98% | Yes | Screening, monitoring treatment | | FTA-ABS (treponemal) | 85–95% | 95% | No (remains positive) | Confirmatory, lifelong positivity | | Dark-field microscopy | >95% | 100% | N/A | Direct organism detection, earliest | **Clinical Pearl:** In the patient described, the classic painless indurated ulcer (chancre) with clean base and regional lymphadenopathy is pathognomonic for primary syphilis. Dark-field microscopy will confirm it by directly visualizing motile spirochetes. **Warning:** Serological tests (VDRL, RPR, FTA-ABS) may be NEGATIVE in the first 1–2 weeks of primary syphilis, making them unreliable for very early diagnosis. Do not rely on them alone in early primary syphilis. [cite:Park 26e Ch 32]
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