In the WHO operational classification of leprosy, which of the following is used as the primary criterion for distinguishing paucibacillary from multibacillary disease?

Explanation

## WHO Operational Classification of Leprosy The WHO introduced a simplified **operational classification** to guide treatment decisions in field/resource-limited settings, replacing the more complex Ridley-Jopling system for public health purposes. ### WHO Classification Criteria | Criterion | Paucibacillary (PB) | Multibacillary (MB) | |-----------|-------------------|-------------------| | **Number of skin lesions** | 1–5 | >5 (6 or more) | | **Number of nerves involved** | ≤1 | >1 | | **Slit-skin smear (SSS)** | Negative | Positive (≥1+) | | **Treatment duration** | 6 months MDT | 12 months MDT | **Key Point:** The **number of skin lesions** is the **primary criterion** in the WHO operational classification. PB leprosy = 1–5 skin lesions; MB leprosy = 6 or more skin lesions. This simple count-based rule was designed to be applicable even in settings where laboratory facilities are unavailable. **High-Yield:** The WHO operational classification was specifically designed as a *field tool* — it deliberately prioritizes the **clinical count of skin lesions** over bacteriological testing. The slit-skin smear (SSS), while useful when available, is NOT the primary criterion; it is an adjunct. In fact, the 1997 WHO Expert Committee explicitly moved away from SSS as the primary criterion to make classification accessible in resource-poor settings (Park's Textbook of Preventive and Social Medicine; WHO Technical Report Series 874, 1998). **Clinical Pearl:** If SSS is positive (≥1+), the patient is automatically classified as MB regardless of lesion count. However, the *primary* and *defining* operational criterion remains the **number of skin lesions**, not the SSS. This distinction is critical for NEET PG and INI-CET. **Mnemonic:** **"5 or fewer = PB; 6 or more = MB"** — count the lesions first; that is the WHO operational rule. *(Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.; WHO Leprosy Elimination Programme guidelines)*