How to Diagnose and Differentiate Scabies
Scabies should be diagnosed by light microscopy demonstration of mites, eggs, or mite feces on mineral oil preparations of multiple skin scrapings, with proper technique requiring vigorous scraping of 20 papules to maximize diagnostic yield. 1
Clinical Recognition
Consider scabies in any patient with an unexplained generalized rash, particularly in long-term care facility residents. 1 The clinical presentation varies significantly by patient population:
Classic Scabies Presentation
- Erythematous papular eruption with serpiginous burrows and intense nocturnal pruritus affecting finger webs, wrists, lateral fingers, elbows, knees, waist, navel, abdomen, buttocks, and genitals 2, 3
- Pruritus is characteristically worse at night 2
- In men, genital involvement is common 2
Atypical Presentations in Debilitated/Elderly Patients
The clinical presentation in nursing home patients is often atypical and diagnostically challenging. 4 Key differences include:
- Burrows, inflammatory changes in intertriginous areas, and pruritus may be completely absent 4
- Debilitated patients may present only with hyperkeratosis, papules, or vesicles 4
- Diagnosis is often made retrospectively when healthcare workers or visitors develop the typical rash 4
- Debilitated older adults are often heavily infested with scabies mites 4
Definitive Diagnostic Technique
Proper Scraping Method
The Infectious Diseases Society of America recommends the following specific technique 1:
- 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 the side of the papule to push the mite out of the burrow 1
- Transfer the oil and scrapings onto a glass slide 1
- For best results, scrape 20 papules 1
Microscopic Examination
- Examine under light microscopy for mites, eggs, or mite feces (scybala - black or brown football-shaped masses) 1, 5
- Finding any of these elements confirms the diagnosis 1, 3
Differential Diagnosis Considerations
Key Distinguishing Features from Other Conditions
Herpes Zoster:
- Presents as painful vesicular rash in dermatomal distribution 4
- Giant cells on Tzanck smear are diagnostic for herpes virus infection 4
- Speciation confirmed by immunofluorescence antigen testing, culture, or PCR of vesicle fluid 4
Herpes Simplex:
- Vesicles or ulcerations involving nasolabial, genital, or rectal skin/mucosa 4
- Tzanck smear shows giant cells 4
Lice Infestation:
- Acquired from infected persons or contaminated fomites like scabies, but organisms are visible to naked eye 4
Critical Diagnostic Pitfalls
Pseudo-Outbreaks
An incorrect diagnosis can lead to pseudo-outbreaks with widespread "psychogenic" scabies (itching but no skin lesions). 4 This occurs when:
- Diagnosis is made on clinical findings alone without microscopic confirmation 4
- Healthcare workers develop anxiety-related pruritus without actual infestation 4
When to Seek Consultation
If proper diagnostic equipment is not available or clinical experience with scabies is limited, consult a dermatologist to inspect or obtain scrapings from suspected persons. 1
Outbreak Recognition
Scabies outbreaks are usually suspected by the occurrence of one unexplained rash in residents of institutional settings. 4
Alternative Diagnostic Methods
When standard scraping is not feasible:
- Epiluminescence microscopy (dermoscopy) can be used as a newer diagnostic technique 2
- High-resolution digital photography may assist in documentation 6
- Skin biopsy can be performed in atypical cases 6
- Clinical diagnosis alone may be necessary in institutional settings, but carries risk of pseudo-outbreaks 4