Which feature best distinguishes paucibacillary (PB) leprosy from multibacillary (MB) leprosy in terms of MDT regimen selection?

Explanation

## Classification Basis for MDT Regimen Selection **Key Point:** The WHO classification of leprosy into paucibacillary (PB) and multibacillary (MB) for the purpose of **MDT regimen selection** is primarily based on the **number of skin lesions** — a simple, field-applicable clinical criterion that does not require laboratory infrastructure. ### Number of Skin Lesions: The WHO Operational Criterion **High-Yield:** - **Paucibacillary (PB) leprosy**: 1–5 skin lesions → 6-month MDT (Rifampicin + Dapsone) - **Multibacillary (MB) leprosy**: **More than 5 skin lesions** → 12-month MDT (Rifampicin + Dapsone + Clofazimine) This criterion was adopted by WHO in 1998 and is the **operational standard** used globally for MDT regimen selection, especially in resource-limited settings where slit-skin smear (SSS) facilities may not be available. ### MDT Regimen Comparison | Feature | PB Leprosy | MB Leprosy | |---------|-----------|----------| | **Skin lesions** | 1–5 | > 5 | | **MDT duration** | 6 months | 12 months | | **Drugs** | Rifampicin + Dapsone | Rifampicin + Dapsone + Clofazimine | | **Supervised dose** | Monthly Rifampicin 600 mg | Monthly Rifampicin 600 mg + Clofazimine 300 mg | | **Daily dose** | Dapsone 100 mg | Dapsone 100 mg + Clofazimine 50 mg | ### Why Not Bacillary Index (BI)? The Bacillary Index (BI) on slit-skin smear is a **laboratory-based** criterion that was historically used for classification. However, WHO's current operational guidelines for MDT regimen selection rely on **lesion count**, not BI, because: 1. **Field applicability**: Lesion counting requires no laboratory — it can be done at any health post. 2. **WHO 1998 revision**: WHO formally shifted to the 5-lesion cutoff as the primary criterion for MDT allocation. 3. **BI ≤ 2+ is not the MDT selection criterion**: BI is used for research/epidemiological purposes; the MDT regimen is assigned based on lesion count in routine practice. 4. **Sensitivity**: Using lesion count avoids under-treatment of smear-negative MB patients. **Clinical Pearl:** A patient with 6 skin lesions but a BI of 1+ is still classified as **MB** for MDT purposes and receives 12-month triple therapy — because the WHO operational criterion is lesion count, not BI. **Mnemonic:** **"5 or fewer = PB (6 months); More than 5 = MB (12 months)"** [cite: Park's Textbook of Preventive and Social Medicine, 26th ed., Ch. 8; WHO MDT Guidelines 1998/2018]