How do you diagnose scabies?

How to Rule Out Scabies

The definitive way to rule out scabies is through light microscopy demonstration of mites, eggs, or mite feces on mineral oil preparations of multiple skin scrapings—if these are absent after proper technique, scabies is effectively ruled out. 1, 2

Clinical Suspicion: When to Consider Scabies

Before attempting to rule out scabies, recognize the clinical scenarios that warrant investigation:

• Consider scabies in any patient with an unexplained generalized rash, particularly in long-term care facility residents 1, 2
• Intense nocturnal pruritus affecting all body regions except the head, appearing out of proportion to physical findings, especially when close contacts are also itching, is virtually diagnostic 3
• Look for burrows in finger webs—the pathognomonic sign of scabies 4, 3

Definitive Diagnostic Technique (Gold Standard)

To properly rule out scabies, perform microscopic examination using the following technique:

Proper Scraping Method

• 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
• For best diagnostic yield, scrape 20 papules to maximize sensitivity 1, 2

What You're Looking For

• Mites, eggs, or mite feces (scybala—black or brown football-shaped masses) on light microscopy 1, 2, 5
• If none of these are found after proper technique on multiple scrapings, scabies is effectively ruled out 1, 2

Critical Diagnostic Pitfalls

Beware of making clinical diagnosis alone without microscopic confirmation—this leads to pseudo-outbreaks with widespread "psychogenic" scabies and unnecessary treatment of healthcare workers who develop anxiety-related pruritus without actual infestation 2

Special Populations with Atypical Presentations

• Debilitated or nursing home patients often lack burrows and may present only with hyperkeratosis, papules, or vesicles 2
• Crusted scabies patients may not experience itching at all 3
• Diagnosis in these populations is often made retrospectively when healthcare workers develop typical rash 2

When Microscopy Is Not Available

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

Alternative Diagnostic Methods

• Video-dermatoscopy or epiluminescence microscopy can visualize mites 6, 7
• Burrow ink test may help identify burrows 7
• Serologic tests (PCR/ELISA, specific IgE) are emerging but not standard practice 7

Differential Diagnosis to Consider

When ruling out scabies, also consider:

• Atopic eczema—look for flexural involvement, family history of atopy, and general dry skin 4
• Lice infestation—organisms are visible to the naked eye 2
• Contact dermatitis—may develop in previously stable eczema 4
• Herpes zoster—painful vesicular rash in dermatomal distribution 2