Skin Scraping (Scabies)

Skin scraping examines material from a scabies burrow for Sarcoptes scabiei mites, eggs and faecal pellets (scybala). Scabies is usually diagnosed clinically, and microscopy confirms the diagnosis when it is uncertain. The clinical picture includes intense itch, often worse at night, a typical distribution and contacts with similar symptoms.

Microscopy has a low yield, so a negative result does not exclude scabies.

Indications. Diagnostic uncertainty, crusted scabies, treatment failure, and outbreak investigation.

Preparation and technique. Burrows on the finger webs, wrists, feet and genital areas give the best yield. Dermoscopy helps identify the burrow, which often shows a delta wing jet sign. Mineral oil is placed on the lesion. The burrow is scraped with a scalpel blade, and the material is transferred to a slide, covered and examined at 100 times to 400 times magnification. Each scraping is documented with the site, the number of mites, eggs and scybala, and the magnification used.

Interpretation.

• Positive: mites, eggs or scybala are seen.
• Crusted scabies: large numbers of mites are present in thick, hyperkeratotic plaques. These cases are highly contagious and need specialist input and isolation measures.

Causes of a false negative result. Mites are rarely found in eczematous skin, and scrapings from unaffected skin miss the burrow.

Limitations and artefacts. A negative scraping in a typical clinical picture does not rule out scabies. Mineral oil can obscure small structures if applied in excess.

Follow up tests. Repeat scraping from a fresh burrow is considered when the clinical picture is typical and the first result was negative. Dermoscopy of the burrow supports the diagnosis when microscopy is unavailable.

Special groups. Infants often have head, neck, palm and sole involvement. Immunosuppressed patients may have atypical presentations, including crusted disease. Pregnancy does not prevent diagnosis by scraping.