A 28-year-old woman with a 10-year history of atopic dermatitis presents with acute worsening of her rash, fever, and pustules over previously affected areas. She reports recent contact with a person who had herpes simplex infection. Which investigation is most appropriate to exclude secondary infection in this patient?

Explanation

## Secondary Infection in Atopic Dermatitis ### Clinical Context: Eczema Herpeticum **Key Point:** Patients with atopic dermatitis have a **severely impaired skin barrier** and are at high risk for secondary infections, including viral (herpes simplex virus [HSV], varicella-zoster virus) and bacterial (Staphylococcus aureus, Streptococcus pyogenes) superinfections. **High-Yield:** The combination of: - Acute worsening of AD - Fever and systemic symptoms - Recent contact with HSV-infected person - Vesicular/pustular lesions ...strongly suggests **eczema herpeticum** — a serious complication requiring urgent antiviral therapy. ### Diagnostic Approach for Suspected Viral Superinfection | Investigation | Indication | Sensitivity | Timing | |---|---|---|---| | **Viral culture + PCR (HSV)** | Suspected eczema herpeticum | High (especially PCR) | Early lesions preferred | | Bacterial culture | Suspected bacterial superinfection (pustules, purulence) | Moderate to high | Any stage | | Serum IgE / specific IgE | Identify allergen triggers | Low specificity for AD diagnosis | Not for acute infection | | Skin biopsy | Chronic AD diagnosis, rule out mimics | High specificity | Not for acute infection | **Clinical Pearl:** **Eczema herpeticum is a medical emergency.** Diagnosis must be made quickly so that IV acyclovir can be initiated. PCR is the most sensitive and rapid test for HSV detection, especially in early vesicular lesions. ### Why Viral Culture + PCR Is Correct 1. **Vesicular fluid** is the ideal specimen — high viral load in early lesions. 2. **PCR** provides rapid, sensitive detection (>95% sensitivity) within hours. 3. **Viral culture** confirms the diagnosis and allows antiviral susceptibility testing. 4. **Positive result** mandates immediate IV acyclovir to prevent dissemination and systemic complications (encephalitis, hepatitis). **Mnemonic: VESICLES in AD = Viral Examination Swab In Cleared Lesions Early Speeds diagnosis** ### Why Bacterial Culture Is Second-Best While bacterial superinfection is common in AD, the clinical presentation here (fever, recent HSV contact, vesicular lesions) is more consistent with viral infection. Bacterial culture would be indicated if pustules with purulence were the primary finding without vesicles.