# Laboratory and functional tests in leprosy diagnosis: a practical guide for clinical decision-making

## TL;DR

This review provides a practical guide on using laboratory and functional tests to support clinical decisions in diagnosing and managing leprosy.

## Contribution

The paper offers a comprehensive synthesis of diagnostic methods for leprosy, emphasizing their practical application in clinical settings.

## Key findings

- Slit-skin smear microscopy is highly specific but has limited sensitivity in paucibacillary leprosy.
- Peripheral nerve ultrasonography is more sensitive than palpation for detecting nerve thickening.
- An integrated approach combining multiple tests is needed to differentiate post-treatment complications.

## Abstract

Early diagnosis of leprosy remains primarily clinical, based on recognition of skin lesions with altered sensation and peripheral nerve involvement. However, complementary tests play crucial roles in confirming uncertain cases, guiding operational classification, detecting subclinical neural involvement, and distinguishing late reactions from relapse. This review synthesizes the practical application of diagnostic methods available to dermatologists. Slit-skin smear microscopy demonstrates high specificity but limited sensitivity in paucibacillary forms, while histopathology reveals the characteristic immunopathological spectrum, with perineural acid-fast bacilli being pathognomonic for leprosy. Molecular detection by PCR enhances diagnosis in paucibacillary cases (34%‒80% sensitivity) but cannot distinguish viable from non-viable bacilli, limiting its utility in post-treatment assessment. Anti-PGL-1 serology aids contact surveillance, with seropositive individuals showing 3.5-fold increased risk of developing disease, though sensitivity remains below 30% in tuberculoid forms. For neural evaluation, Semmes-Weinstein monofilament testing provides a standardized tactile threshold assessment, while the histamine test maps autonomic dysfunction, particularly valuable in indeterminate forms. Electrodiagnostic studies reveal early subclinical changes and monitor reaction-related neural deterioration. Peripheral nerve ultrasonography demonstrates superior sensitivity over palpation for detecting thickening (97.4% vs. 30% concordance) and identifies inflammatory activity through Doppler assessment. When evaluating post-treatment complications, an integrated approach combining bacteriological index trajectory, histopathological patterns, PCR cycle threshold values, and serological trends enables reliable differentiation between therapeutic failure, late reactions, and relapse. No single laboratory test confirms early leprosy in isolation; clinical dermato-neurological expertise remains paramount, with complementary tests interpreted within the epidemiological context to optimize diagnostic accuracy and therapeutic decisions.