What is the proper procedure for performing a skin scraping to diagnose scabies?

Proper Procedure for Skin Scraping to Diagnose Scabies

To diagnose scabies, collect skin scrapings from areas with visible burrows using a sterile scalpel blade with mineral oil, focusing on finger webs, wrists, and other affected areas, then examine under microscopy for mites, eggs, or feces.

Site Selection for Skin Scraping

• Choose areas with visible burrows, papules, or vesicles, particularly:
  • Finger webs
  • Wrists
  • Axillary folds
  • Abdomen
  • Buttocks
  • Genitalia in men 1, 2
• Select active-appearing lesions rather than nearly healed ones 1
• Target the edges of lesions for optimal diagnostic yield 1

Materials Required

• Sterile scalpel blade or dermal curette
• Glass microscope slides
• Coverslips
• Mineral oil (preferred) 1
• Gloves
• Light microscope

Step-by-Step Procedure

1. Preparation:

   • Cleanse the selected area to remove debris and exudates 1
   • Apply a drop of mineral oil directly to the lesion to facilitate adherence of the organisms 1

2. Scraping Technique:

   • Hold the scalpel blade or curette perpendicular to the skin
   • Gently scrape the lesion without causing bleeding 1
   • Focus on scraping the burrow, including its edges and base
   • For fingernail specimens: Use a blunt-tip instrument to gently scrape under the fingernail bed (alternative collection site) 3

3. Sample Collection:

   • Transfer the scraped material onto a glass slide containing a drop of mineral oil
   • Apply a coverslip

4. Examination:

   • Examine under light microscopy (10x and 40x objectives)
   • Look specifically for:
     • Mites (adults or larvae)
     • Eggs
     • Fecal pellets (scybala) 1, 4

Important Considerations

• Timing: Transport specimens to the laboratory within 1 hour for optimal results 1
• Multiple Samples: Collect several scrapings from different lesions to increase diagnostic yield 1
• False Negatives: Be aware that sensitivity of skin scraping is approximately 43-45% 5
• Alternative Method: Standard superficial skin biopsy (SSSB) has higher sensitivity (57%) compared to traditional skin scraping and should be considered if available 5

Pitfalls to Avoid

• Scraping too deeply and causing bleeding, which makes identification of mites more difficult 1
• Using alcohol for skin preparation and not allowing it to dry completely, which can interfere with mite identification
• Collecting samples from excoriated or secondarily infected areas
• Failing to examine multiple sites when initial scrapings are negative
• Not using mineral oil, which is crucial for adherence of organisms 1

Special Populations

• Elderly/Debilitated Patients: May present with atypical distribution including scalp involvement; consider examining these areas 2
• Immunocompromised Patients: For crusted (Norwegian) scabies, collect samples from hyperkeratotic areas where mite burden is extremely high 2

When clinical suspicion remains high despite negative scrapings, consider dermatology consultation for assistance with specimen collection or alternative diagnostic approaches 1.