A 38-year-old man from rural Maharashtra presents with hypopigmented macules on his face and arms with loss of sensation. On examination, no thickened nerves are palpable. Slit-skin smear from the lesion is negative for acid-fast bacilli. Which is the investigation of choice to confirm the diagnosis of leprosy in this patient?

## Diagnosis of Leprosy: Investigation Hierarchy **Key Point:** In a patient with clinical features suggestive of leprosy but negative slit-skin smear, **nerve biopsy with histopathology** is the gold standard confirmatory test. ### Why Nerve Biopsy is the Investigation of Choice Leprosy is fundamentally a disease of peripheral nerves. When slit-skin smear is negative (as in tuberculoid and borderline tuberculoid forms), histopathological examination of an affected nerve or skin lesion showing: - Granulomatous inflammation - Acid-fast bacilli (AFB) within granulomas or macrophages - Nerve involvement with demyelination ...provides definitive diagnosis [cite:KD Tripathi 8e Ch 57]. ### Role of Other Investigations | Investigation | Utility | Limitation | |---|---|---| | **Slit-skin smear** | High bacterial load cases (lepromatous) | Negative in tuberculoid & BT forms | | **Lepromin test (Mitsuda)** | Prognostic/classification tool | NOT diagnostic; appears late; negative in LL/BL | | **Repeated SSS** | May increase yield slightly | Still unreliable in paucibacillary disease | | **PCR for M. leprae** | Sensitive, research setting | Not routinely available; not standard of care | **Clinical Pearl:** The lepromin test becomes positive only after 3–4 weeks and reflects **cell-mediated immunity**, not active infection. It is used for **classification** (tuberculoid = positive; lepromatous = negative), not diagnosis. **High-Yield:** In **paucibacillary leprosy** (tuberculoid, borderline tuberculoid, indeterminate), slit-skin smear is often negative. Diagnosis rests on **clinical features + histopathology showing granulomas and/or AFB in nerve/skin**. ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion of leprosy]:::outcome --> B[Slit-skin smear]:::action B --> C{AFB positive?}:::decision C -->|Yes| D[Lepromatous/BL form<br/>Diagnosis confirmed]:::outcome C -->|No| E[Paucibacillary suspected]:::outcome E --> F[Skin/nerve biopsy<br/>with histopathology]:::action F --> G{Granulomas ±<br/>AFB seen?}:::decision G -->|Yes| H[Leprosy confirmed<br/>Classify by Ridley-Jopling]:::outcome G -->|No| I[Consider other diagnoses]:::outcome ``` **Mnemonic: LEPROSY DIAGNOSIS — "SSB"** - **S** = Slit-skin smear (first-line, high bacterial load) - **S** = Skin/nerve biopsy (gold standard when smear negative) - **B** = Bacillary index on histology (prognostic)