How to Diagnose Scabies
The diagnosis of scabies is primarily clinical, based on characteristic burrows, pruritic rash in typical body locations, and nocturnal itching, with definitive confirmation by microscopic identification of mites, eggs, or fecal pellets (scybala). 1
Clinical Diagnosis Algorithm
Step 1: Assess Classic Clinical Features
The clinical diagnosis can be made when the following triad is present:
- Characteristic lesions: Look for burrows (serpiginous linear tracks), erythematous papules, vesicles, pustules, and excoriations 1, 2
- Typical distribution: Examine finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, periumbilical region, and male genitalia 2, 3
- Intense nocturnal pruritus: The hallmark symptom that distinguishes scabies from other dermatoses 2, 4
Step 2: Recognize Atypical Presentations
In debilitated or elderly patients, the presentation may be atypical—burrows and pruritus may be absent, with only hyperkeratosis, papules, or vesicles present. 1 This is critical because:
- Elderly and immunocompromised patients may present with crusted (Norwegian) scabies, characterized by thick hyperkeratotic crusts harboring thousands to millions of mites 5, 6
- These patients often lack the urge to scratch and therefore do not exhibit typical excoriations 6
- Crusted scabies is far more contagious than typical scabies due to the massive mite burden 5
Step 3: Obtain Definitive Microscopic Confirmation
Definitive diagnosis requires direct visualization of mites, eggs, or fecal pellets on microscopic examination of skin scrapings. 1, 2
The diagnostic procedure:
- Scrape suspected burrows or papules with a #15 scalpel blade after applying mineral oil 2
- Transfer material to a glass slide and examine under light microscopy 2, 3
- Look for the mite itself, oval eggs, or black/brown football-shaped fecal pellets (scybala) 3, 6
Step 4: Consider Alternative Diagnostic Methods
When microscopy is unavailable or negative but clinical suspicion remains high:
- Dermatoscopy (epiluminescence microscopy): A newer technique that can visualize mites in vivo without scraping 2
- Burrow ink test: Apply ink over suspected burrows, wipe off excess—ink tracks into burrow if present 7
- Serologic tests: PCR/ELISA and specific IgE tests are emerging but not yet standard practice 7
Critical Diagnostic Pitfalls to Avoid
- Do not rely solely on microscopy: A negative scraping does not rule out scabies, as the sensitivity is limited—clinical diagnosis alone may be necessary, especially in outbreak settings 1
- Beware of pseudo-outbreaks: Incorrect diagnosis can lead to widespread "psychogenic" scabies (itching without actual infestation) in institutional settings, where anxiety causes symptomatic spread 1
- Do not miss crusted scabies: High clinical suspicion is essential in debilitated, immunocompromised, or elderly patients who may lack typical symptoms but harbor massive mite burdens 1, 6
- Consider the contact history: Transmission occurs through prolonged skin-to-skin contact with relatives, sexual partners, schoolchildren, or in crowded institutional settings 2, 4
When Clinical Diagnosis Alone Is Sufficient
In outbreak settings or when typical clinical features are present with appropriate epidemiologic context (close contact exposure), treatment can be initiated based on clinical diagnosis without waiting for microscopic confirmation. 1 This prevents delays in treatment and reduces transmission risk, particularly in institutional settings where rapid intervention is critical. 6