Lab

Dark-field Exam

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Dark-field Exam, Dark-field Microscopy, Syphilis Microscopy

  • See Also
  1. Syphilis
  2. Syphilis Testing
  • Indications
  1. Syphilis evaluation
  • Efficacy
  1. Most specific if Chancre or condyloma is present
  2. Accuracy varies with experience of technician
  • Equipment Needed
  1. Microscope with dark-field condenser or
  2. Phase contrast microscope
  • Technique
  1. Obtain Sample
    1. Lymph Node saline aspirate
    2. Ulcerative lesion
      1. Clean lesion with saline
      2. Abrade lesion with gauze
      3. Do not use antiseptic
      4. Squeeze lesion to obtain drop of serous transudate
  2. Place drop of sample onto glass slide; add cover slip
  3. Examine under dark-field or phase contrast microscope
    1. Single motile organism (corkscrew appearance)
  • Interpretation
  • Positive Test
  1. Primary Syphilis Chancre
  2. Secondary Syphilis Condyloma Lata
  • Interpretation
  • Negative Test
  1. Does not rule out Syphilis
  2. At least 10^4 organisms must be present to be seen
  3. Repeat exam on three consecutive days
  • References
  1. Larsen (1995) Clin Microbiol Rev 8:1-21 [PubMed]