How to Diagnose Scabies
Scabies should be diagnosed definitively by light microscopy demonstration of mites, eggs, or mite feces on mineral oil preparations of multiple skin scrapings, using proper scraping technique on at least 20 papules to maximize diagnostic yield. 1, 2
When to Suspect Scabies
- Consider scabies in any patient with an unexplained generalized pruritic rash, particularly in long-term care facility residents, immunocompromised individuals, or those with close contact exposure 1, 2
- The pathognomonic sign is burrows in finger webs, though these may be absent in atypical presentations 1
- Classic presentation includes burrows, erythematous papules, and intense nocturnal pruritus affecting finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, and male genitalia 3, 4
- Elderly and debilitated patients often present atypically without burrows, showing only hyperkeratosis, papules, or inflammatory changes in intertriginous areas 2, 5
Definitive Diagnostic Technique: Skin Scraping with Microscopy
The gold standard diagnostic method requires proper scraping technique: 1, 2
- Place a drop of mineral oil on a sterile scalpel blade and allow oil to flow onto the papule 1
- Scrape vigorously 6-7 times to remove the top of the papule 1
- Use the flat side of the scalpel to apply pressure to push the mite out of the burrow 1
- Transfer the oil and scrapings onto a glass slide for microscopic examination 1
- Scrape at least 20 papules to maximize diagnostic yield 1, 2
Critical Limitation of Microscopy
- False negatives are extremely common—negative microscopy does NOT exclude scabies, as only 10-20% of cases show visible mites on examination 1
- This low sensitivity means clinical diagnosis may be necessary in practice, though it carries risk of pseudo-outbreaks 2
Alternative and Adjunctive Diagnostic Methods
Dermoscopy
- Dermoscopy is increasingly used in clinical practice and is more effective than microscopy alone (10.7% positive rate vs 6.67% for microscopy) 6, 7
- Can visualize burrows and mites without requiring scraping 6
PCR Testing
- Real-time PCR from skin scrapings detects 28% of cases, and combined scraping plus wet skin swabs detects 36.6% 7
- PCR is more sensitive than microscopy and correlates with pruritus severity and body surface area involvement 7
- All microscopy-positive results are also PCR-positive, confirming PCR's superior sensitivity 7
When Expertise is Limited
- If proper diagnostic equipment is unavailable or clinical experience with scabies is limited, consult a dermatologist to inspect or obtain scrapings from suspected persons 1, 2
Differential Diagnosis Considerations
Key conditions to distinguish from scabies include:
- Atopic eczema: flexural involvement, family history of atopy, generalized dry skin 1
- Contact dermatitis: may develop in previously stable eczema 1
- Herpes zoster: painful vesicular rash in dermatomal distribution, diagnosed by Tzanck smear showing giant cells 2
- Herpes simplex: vesicles or ulcerations in nasolabial, genital, or rectal areas 2
- Lice infestation: organisms visible to naked eye 2
Critical Diagnostic Pitfalls to Avoid
- Pseudo-outbreaks occur when diagnosis is made on clinical findings alone without microscopic confirmation, leading to widespread unnecessary treatment for psychogenic pruritus 2, 5
- In institutional settings, one unexplained rash case should trigger outbreak investigation 2
- Diagnosis is often made retrospectively when healthcare workers or visitors develop typical rash after exposure to atypically presenting patients 2
- Persistent pruritus for up to 2 weeks after treatment is normal and does NOT indicate treatment failure or ongoing infestation 1, 5
- Retreatment is indicated only if symptoms persist beyond 2 weeks AND living mites are demonstrated on repeat scraping 1, 5