# WHO Classification and Diagnosis of Leprosy

> WHO classification and diagnosis of leprosy for NEET-PG Dermatology: paucibacillary and multibacillary definitions, cardinal signs and field diagnosis steps.

- Canonical URL: https://prepelephant.com/topics/neet-pg/dermatology/who-classification-and-diagnosis-of-leprosy
- Exam / course: NEET-PG · Subject: Dermatology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Park's Textbook of Preventive and Social Medicine; IADVL Textbook of Dermatology; WHO Guidelines for the Diagnosis, Treatment and Prevention of Leprosy
- How to cite: "WHO Classification and Diagnosis of Leprosy", PrepElephant, https://prepelephant.com/topics/neet-pg/dermatology/who-classification-and-diagnosis-of-leprosy

## Direct answer

The WHO operational classification divides leprosy into paucibacillary (PB) disease, with five or fewer skin lesions and a negative slit-skin smear, and multibacillary (MB) disease, with more than five lesions or a positive smear. This category decides the multidrug regimen. Diagnosis is clinical and rests on one or more cardinal signs: a hypopigmented or reddish patch with sensory loss, a thickened peripheral nerve, or a positive skin smear.

## What you must remember

- **Paucibacillary (PB):** five or fewer skin lesions with a negative slit-skin smear, treated with a six-month multidrug regimen.
- **Multibacillary (MB):** more than five skin lesions, or any positive smear, regardless of the lesion count; treated with a twelve-month regimen. India's revised NLEP scheme also places any nerve involvement in MB.
- **Cardinal sign one:** a hypopigmented or reddish skin patch with definite loss of sensation within it.
- **Cardinal sign two:** a thickened peripheral nerve with sensory loss or weakness in the area it supplies.
- **Cardinal sign three:** a positive slit-skin smear for acid-fast bacilli.
- **Diagnostic rule:** one or more cardinal signs are sufficient to diagnose leprosy in field practice.
- **Nerves examined:** greater auricular, ulnar, radial cutaneous, median, lateral popliteal and posterior tibial nerves, each palpated for thickening and tenderness.
- **Differentials:** pityriasis alba, tinea versicolor, vitiligo and post-inflammatory hypopigmentation, which lack sensory loss within the patch.

## Field diagnosis of a suspected case

A hypopigmented or reddish patch in a patient from an endemic area should prompt a structured field examination. The steps below are short enough to complete at a primary health centre, and they lead directly to the operational category.

1. **Lesion survey:** count the skin lesions and record the colour, borders and sites of each patch, because the count determines PB or MB status.
2. **Sensory mapping:** test light touch with cotton wool and pinprick with a sterile pin, comparing each lesion with normal skin, since anaesthesia within a patch is significant.
3. **Nerve palpation:** examine the six named nerves for thickening and tenderness, and test the muscles they supply for weakness or deformity.
4. **Slit-skin smear:** take smears from the earlobes and lesions for Ziehl-Neelsen staining, and record the bacteriological index.
5. **Classification:** combine the lesion count, nerve findings and smear result to assign PB or MB, and start multidrug therapy promptly.
6. **Recording:** register the case, grade any disability, and screen household contacts for the disease.

The classification is operational rather than a measure of severity, so it guides treatment duration and drug choice rather than prognosis. Clinicians should record it clearly at the first visit, because the regimen cannot be chosen correctly without it.

## Where the exam catches students

Most traps involve misapplying the lesion count or treating a patch without testing it.

- **Trap:** classifying a patient with six lesions as paucibacillary — **Correct:** more than five lesions makes the case multibacillary, so the twelve-month regimen applies.
- **Trap:** classifying by lesion count alone — **Correct:** a positive smear makes any case multibacillary, even when only a few lesions are present.
- **Trap:** treating a hypopigmented patch as leprosy without sensory testing — **Correct:** loss of sensation within the patch is needed, so testing decides between leprosy and pityriasis alba or vitiligo.
- **Trap:** treating a negative smear as ruling out leprosy — **Correct:** PB disease is smear-negative, so clinical cardinal signs remain the basis of diagnosis.

## Frequently asked questions

### What is the difference between paucibacillary and multibacillary leprosy?
Paucibacillary leprosy has five or fewer skin lesions and a negative slit-skin smear, while multibacillary leprosy has more than five lesions or a positive smear. The category determines whether a six-month or twelve-month multidrug therapy regimen is given.

### What are the WHO cardinal signs of leprosy?
The WHO cardinal signs of leprosy are a hypopigmented or reddish skin patch with definite loss of sensation, a thickened peripheral nerve with sensory or motor loss, and a positive skin smear for acid-fast bacilli. One cardinal sign is enough for a diagnosis of leprosy in field practice.

### Which nerves are examined in the field diagnosis of leprosy?
Six nerves are examined in field practice: the greater auricular, ulnar, radial cutaneous, median, lateral popliteal and posterior tibial nerves. Each is palpated for thickening and tenderness, and the muscles it supplies are tested for weakness.

### Why must sensation be tested within every hypopigmented patch?
Sensory loss within a patch is a key feature that separates leprosy from pityriasis alba and vitiligo. Light touch with cotton wool and pinprick with a sterile pin, compared with normal skin, establish whether anaesthesia is present.