A 28-year-old man with a 3-week history of intensely pruritic papules and burrows on the hands, wrists, and genitalia is suspected to have scabies. Microscopic examination of scrapings from two burrows is negative. The patient has no systemic symptoms, and family members are asymptomatic. What is the most appropriate next investigation?

Explanation

## Approach to Negative Microscopy in Suspected Scabies **Key Point:** A single negative scraping does not exclude scabies. Repeat scrapings from multiple, carefully selected burrows should be performed before abandoning the diagnosis, as sensitivity of microscopy is only 60–80%. ### Why Repeat Scrapings? **High-Yield:** Sensitivity of scrapings varies with: - **Site selection** — Burrows yield higher positivity than papules; interdigital spaces, wrists, and genitalia are optimal - **Mite burden** — Early infestation or partially treated cases may have fewer organisms - **Technique** — Inadequate scraping or failure to include burrow material reduces yield - **Number of attempts** — Repeating from multiple sites increases cumulative sensitivity to >90% ### Diagnostic Algorithm for Negative Microscopy ```mermaid flowchart TD A["Clinical suspicion of scabies"]:::outcome --> B["Scrape from burrow + microscopy"]:::action B --> C{"Mite/eggs/feces seen?"}:::decision C -->|Yes| D["Confirm diagnosis"]:::outcome C -->|No| E["Repeat scrapings from 3–5 different burrows"]:::action E --> F{"Positive on repeat?"}:::decision F -->|Yes| D F -->|No| G{"High clinical suspicion remains?"}:::decision G -->|Yes| H["Treat empirically or consider dermoscopy/biopsy"]:::action G -->|No| I["Consider alternative diagnosis"]:::outcome ``` ### Technique for Optimal Yield 1. **Select unscratched burrows** — Scratched lesions have fewer mites 2. **Apply mineral oil or 10% KOH** — Softens the burrow and aids mite extraction 3. **Scrape firmly** — Ensure material from the burrow itself, not just surface 4. **Examine entire slide** — Use low power (10×) then high power (40×) systematically 5. **Repeat from multiple sites** — Increases sensitivity significantly **Clinical Pearl:** In the setting of high clinical suspicion (typical distribution, family involvement, nocturnal pruritus, burrows on exam), a negative single scraping should prompt repeat attempts rather than diagnostic abandonment. Many cases are confirmed on the second or third attempt. ### Why Not the Other Options? | Option | Why Inappropriate | |--------|-------------------| | **Skin biopsy** | Invasive and unnecessary as first repeat step; reserved for atypical presentations or after multiple negative scrapings | | **CBC and IgE** | Non-specific; may show eosinophilia in scabies but does not confirm diagnosis | | **Fungal culture** | Scabies is a mite infestation, not fungal; this test is irrelevant and delays diagnosis | **Mnemonic:** **SCRAPE** — **S**elect burrows, **C**lean with oil, **R**ub firmly, **A**pply to slide, **P**ower microscopy, **E**xamine thoroughly. [cite:Irvine's Dermatology 10e Ch 31]