A 28-year-old female presents with circular, scaly patches on the scalp with hair loss (alopecia) for 6 weeks. She reports recent contact with a stray cat. Examination reveals well-demarcated lesions with broken hairs at the surface. Which investigation is most specific for confirming tinea capitis and identifying the causative organism?

Explanation

## Diagnosis of Tinea Capitis **Key Point:** Fungal culture on Sabouraud dextrose agar (SDA) is the most specific investigation for tinea capitis because it allows both organism identification and species differentiation, which guides treatment decisions. ### Why Culture is Superior for Tinea Capitis 1. **Species identification**: Different dermatophytes require different treatment durations (e.g., *Microsporum canis* vs. *Trichophyton tonsurans*). 2. **Antifungal susceptibility testing**: Culture allows assessment of resistance patterns, important for treatment failures. 3. **Definitive diagnosis**: Culture is the gold standard; KOH mount is presumptive only. 4. **Epidemiological tracking**: Helps identify zoonotic vs. anthropophilic sources (contact with cat suggests *M. canis*). ### Specimen Collection for Tinea Capitis **High-Yield:** - **Pluck** (do not cut) 10–15 affected hairs with roots and scales. - Include both affected and perilesional hairs. - Transport in sterile container to mycology lab. - Culture takes 2–4 weeks; KOH mount can be done simultaneously for rapid presumptive diagnosis. ### Comparison of Investigations | Investigation | Sensitivity | Specificity | Speed | Organism ID | Clinical Use | |---|---|---|---|---|---| | **KOH mount** | 70–80% | High | 24 hrs | No | Rapid presumptive diagnosis | | **Fungal culture (SDA)** | 85–95% | Very high | 2–4 weeks | **Yes** | Definitive diagnosis + species ID | | **Direct microscopy** | 50–60% | Moderate | 1 hr | No | Limited utility | | **Bacterial culture** | N/A | N/A | N/A | N/A | Inappropriate for fungal infection | **Clinical Pearl:** Tinea capitis in children is often caused by *Trichophyton tonsurans* (anthropophilic, human-to-human spread), whereas contact with animals (as in this case) suggests *Microsporum canis* or *M. audouinii* (zoophilic). Culture identifies the source and guides public health measures. ### Diagnostic Algorithm for Tinea Capitis ```mermaid flowchart TD A[Suspected tinea capitis]:::outcome --> B[Pluck hairs + scales]:::action B --> C[Perform KOH mount]:::action B --> D[Inoculate SDA culture]:::action C --> E{KOH positive?}:::decision E -->|Yes| F[Presumptive diagnosis: tinea capitis]:::outcome E -->|No| G[Culture result pending]:::action D --> H{Culture positive?}:::decision H -->|Yes| I[Definitive diagnosis + organism ID]:::outcome H -->|No| J[Reconsider diagnosis]:::action I --> K[Tailor treatment by species]:::action ``` **Mnemonic:** **SDA for Species Determination in Alopecia** — Sabouraud dextrose agar identifies the specific dermatophyte causing scalp ringworm.