A 28-year-old woman from Odisha presents with a single hypopigmented, well-demarcated skin lesion on her forearm with loss of sensation. Slit-skin smear is negative for acid-fast bacilli. Clinical suspicion for tuberculoid leprosy remains high. Which investigation would be most appropriate to confirm the diagnosis in this paucibacillary form?

Explanation

## Diagnosis of Paucibacillary Leprosy (Tuberculoid Form) **Key Point:** In paucibacillary leprosy (tuberculoid and borderline tuberculoid), bacillary load is extremely low; slit-skin smears are often negative. **Skin biopsy with histopathology is the investigation of choice** to confirm diagnosis when clinical suspicion is high but smear is negative. ### Why Biopsy in Paucibacillary Disease? 1. **Demonstrates granulomatous inflammation** characteristic of tuberculoid leprosy 2. **Identifies acid-fast bacilli** within granulomas (even if scanty) 3. **Immunohistochemistry** (anti-M. leprae antibodies) increases detection sensitivity 4. **Excludes mimics** (vitiligo, indeterminate dermatitis, other granulomatous conditions) 5. **Definitive histological diagnosis** when smear is negative ### Investigation Strategy in Paucibacillary Leprosy ```mermaid flowchart TD A[Clinical suspicion of leprosy]:::outcome --> B[Slit-skin smear]:::action B --> C{Smear positive?}:::decision C -->|Yes| D[Multibacillary or paucibacillary]:::outcome C -->|No| E{High clinical suspicion?}:::decision E -->|No| F[Reassess diagnosis]:::action E -->|Yes| G[Skin biopsy + histopathology]:::action G --> H[Granulomas + AFB on IHC]:::outcome H --> I[Confirm tuberculoid leprosy]:::outcome ``` ### Comparison: Paucibacillary vs Multibacillary Investigations | Feature | Multibacillary (LL/BL) | Paucibacillary (BT/TT) | |---|---|---| | **Slit-skin smear** | Positive (high BI) | Negative (low/no bacilli) | | **Histology** | Foamy macrophages, many AFB | Epithelioid granulomas, few/no AFB | | **Immunohistochemistry** | Abundant bacilli | Scanty bacilli (IHC helpful) | | **Lepromin test** | Negative | Positive (3–4 weeks) | | **Diagnosis method** | Smear + clinical | **Biopsy + histology** | **High-Yield:** Paucibacillary leprosy (TT/BT) often requires **skin biopsy** because bacillary load is too low for smear detection. Immunohistochemistry with anti-M. leprae antibodies increases diagnostic yield. **Clinical Pearl:** A single hypopigmented, anaesthetic lesion with negative smear in an endemic area is tuberculoid leprosy until proven otherwise. Biopsy showing epithelioid granulomas with few AFB (on IHC) confirms diagnosis. **Mnemonic:** **BIOPSY in PAUCIBACILLARY** = **B**acilli scanty, **I**mmunohistochemistry needed, **O**nly histology shows granulomas, **P**aucibacillary smear negative, **S**kin biopsy is diagnostic, **Y**ield improved with IHC. [cite:Park 26e Ch 7; Harrison 21e Ch 189]