How to diagnose scabies in an adult or elderly patient with a suspected history of skin conditions or immunocompromised status, presenting with a rash or skin lesions?

Diagnostic Imaging and Visual Identification of Scabies

Direct Visualization Techniques

Dermoscopy is the most practical and effective imaging modality for diagnosing scabies in real-time, allowing direct visualization of mites, burrows, eggs, and fecal pellets without requiring laboratory processing. 1, 2

Dermoscopic Findings (Primary Diagnostic Approach)

• Look for the pathognomonic "delta wing sign" - a triangular dark structure representing the mite's head and anterior legs at the end of a burrow 3
• Identify sinuous burrows appearing as dark, wavy lines filled with white dots (eggs and fecal pellets) 4
• The mite itself appears as a small dark triangle at the terminal end of the burrow 4, 3
• Dermoscopy-guided scraping is superior to blind scraping in both duration and diagnostic accuracy, particularly in patients previously treated with steroids (scabies incognito) 3

Clinical Photography

• High-resolution digital photography can document burrows and lesion distribution patterns, particularly useful for tracking treatment response 5
• Standard clinical photographs should capture characteristic distribution: finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, and male genitalia 6
• In elderly and immunocompromised patients, photograph the scalp, hairline, neck, temple, and forehead as these areas are frequently involved 7, 2

Advanced Imaging Modalities

Reflectance Confocal Microscopy (RCM)

• RCM provides real-time, noninvasive visualization of mites and burrows at cellular resolution without requiring skin scraping 4
• Particularly valuable in crusted scabies where mite burden is extremely high and diagnosis is critical 4
• This technique is not widely available but represents the gold standard for noninvasive diagnosis when accessible 4

Fluorescence Staining

• Fluorescent dyes can highlight mites in skin scrapings or in vivo, improving visualization 4
• This adjunctive technique is primarily used in research settings rather than routine clinical practice 4

Critical Diagnostic Pitfalls in Imaging

• The absence of visible burrows does NOT exclude scabies, especially in elderly, debilitated, or immunocompromised patients who may present only with hyperkeratosis, papules, or atypical inflammatory changes 2, 8
• Negative microscopy occurs in 80-90% of cases due to low mite burden in typical scabies (only 10-20 mites per patient), making dermoscopy essential for improving diagnostic yield 1
• In nursing home residents, clinical presentation is often atypical without classic burrows, showing only intertriginous inflammation and pruritus 2

Practical Algorithm for Visual Diagnosis

1. First-line approach: Perform dermoscopy on suspected lesions, focusing on finger webs, wrists, and other characteristic sites 1, 3
2. If burrows are visible on dermoscopy: This is pathognomonic and confirms diagnosis without requiring scraping 1, 3
3. If dermoscopy is unavailable or inconclusive: Obtain dermatology consultation for expert dermoscopic examination or skin scraping 1, 2
4. For immunocompromised or elderly patients: Examine and photograph scalp, face, and neck in addition to typical sites 7, 2
5. Document extent of involvement using body surface area mapping, particularly important in crusted scabies 9

Special Populations Requiring Enhanced Imaging

• Crusted (Norwegian) scabies in immunocompromised patients presents with hyperkeratotic plaques and extremely high mite burden - dermoscopy will show numerous mites easily 1, 4
• Institutional outbreak settings require systematic photographic documentation to track spread and treatment response 2, 8
• Post-treatment evaluation: Persistent lesions at 2 weeks warrant repeat dermoscopy to distinguish treatment failure from normal post-scabetic dermatitis 1, 8