Micro & Immunology · single question
Which of the following is the MOST appropriate test to diagnose active syphilis infection?
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Correct answer: B. NAAT (nucleic acid amplification test)
The nucleic acid amplification test (NAAT) is the most sensitive and specific test for diagnosing active Treponema pallidum infection, especially in oral or extragenital lesions where darkfield microscopy may be difficult. NAAT detects treponema DNA directly. RPR and VDRL are non-treponemal serologic screening tests (can have false positives). Treponemal tests (FTA-ABS, TPPA) confirm syphilis but don't distinguish active from past infection.
- A. ❌ RPR is a non-treponemal screening test with potential false positives (pregnancy, autoimmune disease); it cannot definitively diagnose active infection.
- B. ✅ CORRECT – NAAT directly detects T. pallidum DNA and is the most specific test for active syphilis, especially in oral lesions.
- C. ❌ VDRL is another non-treponemal test similar to RPR — useful for CNS syphilis screening but not the most specific.
- D. ❌ Darkfield microscopy can identify treponemas directly from lesion exudate but is technically difficult and requires specialized equipment.
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