A 35-year-old man from rural Maharashtra presents with hypopigmented macules on his face and trunk with loss of sensation. Slit-skin smear examination shows acid-fast bacilli. Which investigation is most appropriate to classify the type of leprosy and guide MDT regimen selection?

Explanation

## Classification and MDT Regimen Selection in Leprosy ### Role of Bacterial Index (BI) **Key Point:** The bacterial index (BI) on slit-skin smear is the primary investigation for classifying leprosy type and determining MDT regimen duration. - BI is graded on a logarithmic scale (0 to 6+) - BI ≥ 2 indicates **lepromatous leprosy (LL)** or **borderline lepromatous (BL)** → **12-month MDT** - BI < 2 indicates **tuberculoid (TT)**, **borderline tuberculoid (BT)**, or **mid-borderline (BB)** → **6-month MDT** ### Why BI is Superior for MDT Guidance | Investigation | Purpose | Role in MDT Selection | |---|---|---| | **Bacterial Index** | Quantifies bacillary load | **Direct indicator of disease burden and treatment duration** | | Lepromin test | Assesses cell-mediated immunity | Classifies type but does NOT guide MDT duration | | Histopathology | Confirms diagnosis and type | Useful but slower; BI is faster and more practical | | PCR | Detects M. leprae DNA | Research tool; not routine for MDT decisions | **Clinical Pearl:** A BI of 2 or higher on slit-skin smear is the **threshold for 12-month MDT regimen**. This simple, rapid, field-friendly test is why it remains the gold standard in leprosy control programs across India. **High-Yield:** The WHO classification for MDT is based on **clinical signs + BI**, not on lepromin test result. Lepromin is positive in TT and BT (good immunity) and negative in LL (poor immunity), but this does NOT change the MDT regimen — BI does. [cite:Park 26e Ch 8]