## Investigation of Choice for Leprosy Classification **Key Point:** Histopathology of skin biopsy is the gold standard for classifying leprosy into tuberculoid (TT), borderline tuberculoid (BT), mid-borderline (BB), borderline lepromatous (BL), and lepromatous (LL) forms. ### Why Histopathology? Histopathology provides: - **Granuloma architecture** — epithelioid cells (TT/BT) vs foamy macrophages (BL/LL) - **Bacillary load** — morphological assessment of AFB burden - **Epidermal involvement** — perineural and periadnexal inflammation patterns - **Classification certainty** — enables accurate assignment to Ridley-Jopling classification ### Classification Basis | Feature | TT | BT | BB | BL | LL | |---------|----|----|----|----|----| | **Granulomas** | Well-formed | Well-formed | Ill-defined | Poorly formed | Absent | | **Bacillary load** | Rare (0–1) | Few (1–5) | Moderate (4–6) | Numerous (5–6) | Abundant (6) | | **Epidermal involvement** | No | No | Yes | Yes | Yes | | **Histology** | Epithelioid | Epithelioid | Mixed | Foamy macrophages | Foamy macrophages | **High-Yield:** The Ridley-Jopling classification is based on **histopathology**, not clinical features alone. This is why biopsy is the investigation of choice for definitive classification. ### Clinical Pearl In this case, the high AFB load (4+) on slit-skin smear suggests lepromatous or borderline lepromatous disease, but only histopathology can distinguish between them and guide whether the patient needs 12 months (BL) or 24 months (LL) of multidrug therapy (MDT). [cite:Park 26e Ch Leprosy] 
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