A 32-year-old woman presents with intense pruritus, worse at night, affecting the interdigital spaces, flexural surfaces, and genitalia. Multiple family members are similarly affected. Clinical examination reveals burrows and nodules. Which is the investigation of choice to confirm the diagnosis of scabies?

Explanation

## Investigation of Choice for Scabies Confirmation **Key Point:** Potassium hydroxide (KOH) mount of skin scrapings is the gold standard and most practical confirmatory test for scabies in clinical practice. ### Why KOH Mount is the Gold Standard 1. **High sensitivity and specificity** when scrapings are taken from active burrows 2. **Simple, rapid, and cost-effective** — can be performed in outpatient clinic 3. **Direct visualization** of mites, eggs, feces (scybala), or mite fecal pellets under microscope 4. **Non-invasive** — no need for biopsy ### Technique for Maximum Yield - Scrape the **interdigital webs** (highest mite density) - Scrape **burrows** identified clinically or with dermoscopy - Use a blunt blade or curette with mineral oil or KOH solution - Mount on glass slide with 10–20% KOH - Examine under low power (10×) for characteristic mite morphology ### Sensitivity and Yield | Method | Sensitivity | Specificity | Notes | | --- | --- | --- | --- | | KOH mount (burrow) | 60–80% | >95% | Best from burrows; multiple scrapings increase yield | | Dermoscopy | 95–98% | >95% | Non-invasive, rapid; requires dermoscope | | Histopathology | 100% | 100% | Overkill; invasive; reserved for atypical cases | | Bacterial culture | N/A | N/A | Identifies secondary infection, not diagnostic for scabies | **High-Yield:** If KOH mount is negative but clinical suspicion is high, repeat scrapings from multiple sites (burrows, interdigital spaces, wrists) or use dermoscopy as alternative confirmation. **Clinical Pearl:** In Norwegian (crusted) scabies, KOH mount has even higher sensitivity because mite burden is 100–1000× higher than in classical scabies.