What are the diagnostic criteria for leprosy?

Diagnostic Criteria for Leprosy

Leprosy is diagnosed when a patient presents with at least one of three cardinal clinical signs: hypopigmented or erythematous skin patches with definite sensory loss, thickened peripheral nerves with dysfunction, or demonstration of acid-fast bacilli in skin smears or biopsy specimens. 1

Clinical Cardinal Signs

The diagnosis relies on identifying specific clinical features that distinguish leprosy from other dermatological and neurological conditions:

1. Skin Lesions with Sensory Deficit

• Hypopigmented or erythematous patches with definite sensory loss are pathognomonic for leprosy 1, 2
• The sensory deficit must be demonstrable within the skin lesion itself (excluding nodules and infiltration) 2
• Lesions may present as macules, papules, nodules, plaques, or infiltration 2

2. Peripheral Nerve Involvement

• Thickened peripheral nerves with tenderness and/or dysfunction constitute the second clinical cardinal sign 1, 2
• Common cutaneous nerves should be systematically examined for thickening 3
• Nerve involvement is the primary cause of disability in leprosy 4, 5

3. Bacteriological Confirmation

• Demonstration of acid-fast bacilli in slit skin smears or full-thickness skin biopsy from lesions provides laboratory confirmation 1, 2
• Smears should be taken from both ear lobes and at least one lesion when clinical signs are equivocal 2
• Laboratory examination is not essential when two clinical cardinal signs are present 3

Clinical Spectrum and Classification

The disease manifests across a spectrum based on cellular immune response to Mycobacterium leprae: 1, 4

Tuberculoid Form

• One or few well-demarcated, hypopigmented, anesthetic lesions 1
• Active spreading edges with central clearing 1
• Peripheral nerve swelling may occur 1

Lepromatous Form

• Multiple erythematous papules and nodules 1
• Bilateral and symmetrical distribution 1
• Infiltration of face, hands, and feet with progressive skin thickening 1

Borderline (Dimorphous) Form

• Features of both tuberculoid and lepromatous forms 1

Indeterminate Form

• Early hypopigmented macules without developed features 1

Operational Classification for Treatment

For treatment purposes, classify as paucibacillary (≤5 lesions) or multibacillary (≥6 lesions) based on lesion count, margin characteristics, sensory deficit, satellite lesions, symmetry, central healing, and scaling 2

Diagnostic Approach

When leprosy is suspected but only one cardinal sign is present:

• Perform slit skin smear examination from both ear lobes and one lesion to detect acid-fast bacilli 2
• Consider histopathology when clinical and bacteriological findings are inconclusive 6
• History of household contact with leprosy patients provides supportive evidence 3

Critical Pitfalls

• Do not rely on urine or serum antigen tests, as these are unreliable 1
• Sensory testing must be performed systematically on all suspicious lesions 3
• Early diagnosis is paramount to prevent nerve damage and disability, which occurs in less than 2% of cases when treated promptly 2
• The long incubation period (2-4 years) means patients may not recall exposure 2

Confirmation Requirements

A confirmed case requires laboratory demonstration of acid-fast bacilli in skin or dermal nerve specimens from a clinically compatible case 1. However, in resource-limited settings where leprosy is endemic, clinical diagnosis based on two cardinal signs is sufficient to initiate treatment 3.