Diagnostic Approach for Scabies
Scabies diagnosis should be confirmed by light microscopy demonstration of mites, eggs, or mite feces on mineral oil preparations of skin scrapings, though clinical diagnosis alone may be necessary when diagnostic equipment is unavailable. 1, 2
Clinical Presentation and When to Suspect Scabies
Consider scabies in any patient with unexplained generalized rash, particularly in long-term care facility residents. 1, 2 Key clinical features include:
- Intense pruritus, especially nocturnal, though this may be absent in debilitated elderly patients 1
- Burrows (thread-like papules), inflammatory changes in intertriginous areas, though these classic findings may be absent in nursing home patients 1, 3
- Atypical presentations in debilitated older adults: hyperkeratosis, papules, or vesicles without pruritus or burrows 1
- Typical distribution: finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, and genitalia in men 4, 5
Gold Standard Diagnostic Method
The definitive diagnosis requires light microscopy of mineral oil preparations from skin scrapings. 1, 2 Proper technique is critical:
Scraping Technique
- Place a drop of mineral oil on a sterile scalpel blade and allow oil to flow onto the papule 2
- Scrape vigorously 6-7 times to remove the top of the papule 2
- Use the flat side of the scalpel to apply pressure to push the mite out of the burrow 2
- Scrape 20 papules for best results to increase diagnostic yield 2
- Transfer oil and scrapings onto a glass slide for microscopic examination 2
What to Look For
Alternative Diagnostic Methods
Dermoscopy is more sensitive than microscopy alone (10.7% vs 6.67% positive rate) and can be performed at bedside. 6, 3 This technique allows visualization of burrows and mites without scraping.
Real-time PCR from skin scrapings is the most sensitive method (28% positive rate from scrapings alone, 36.6% when combined with wet swabs), though not widely available. 6 All microscopy-positive results were also PCR-positive, confirming microscopy's specificity. 6
Clinical Diagnosis When Laboratory Confirmation Unavailable
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
Clinical diagnosis may be necessary based on:
- Characteristic distribution and appearance of lesions 1
- Occurrence of typical rash in healthcare workers or visitors exposed to the patient, which often triggers outbreak recognition 1
- Epidemiologic context (multiple cases in facility, known exposure) 1
Critical Diagnostic Pitfalls
Avoid making diagnosis based solely on clinical signs without attempting microscopic confirmation, as this leads to pseudo-outbreaks with widespread "psychogenic" scabies (itching without actual infestation). 1 An incorrect diagnosis can result in unnecessary treatment of multiple individuals.
In debilitated older adults, expect heavy mite burden despite minimal symptoms, making microscopic confirmation more likely to succeed. 1
Surface swab cultures are not indicated for scabies diagnosis—this is a parasitic infestation requiring direct visualization of the organism. 1