## Distinguishing Feature: Bacillary Load and Lesion Count **Key Point:** The cardinal discriminator between PB and MB leprosy is the **number of skin lesions and involved nerve trunks**, which directly determines MDT regimen choice and duration. ### Classification Criteria | Feature | Paucibacillary (PB) | Multibacillary (MB) | |---------|-------------------|-------------------| | **Skin lesions** | ≤5 | >5 | | **Nerve trunks involved** | ≤5 | >5 | | **Bacillary index (BI)** | Negative or <1+ | ≥1+ (often 4–6+) | | **Slit-skin smear** | Negative | Positive (2+ to 6+) | | **MDT duration** | 6 months | 12 months | | **Infectivity** | Low | High | **High-Yield:** The WHO classification uses **lesion count and nerve trunk involvement** as the primary operational criterion — not bacillary index or smear positivity — because it is clinically practical and determines treatment intensity. ### MDT Regimens **PB MDT (6 months):** - Rifampicin 600 mg monthly (supervised) + Dapsone 100 mg daily (unsupervised) **MB MDT (12 months):** - Rifampicin 600 mg monthly (supervised) + Dapsone 100 mg daily (unsupervised) + Clofazimine 300 mg monthly (supervised) + 50 mg daily (unsupervised) **Clinical Pearl:** A patient with 6 skin lesions and 2 nerve trunks involved is classified as **MB** (because skin lesions >5), even if smear is negative. This patient receives 12-month MDT, not 6-month. **Mnemonic: "PB ≤5, MB >5"** — Remember the threshold of 5 lesions or 5 nerve trunks as the dividing line. 
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