Diagnosing multibacillary leprosy: a comparative evaluation of diagnostic accuracy of slit-skin smear, bacterial index of granuloma and WHO operational classification.

BACKGROUND: In view of the relatively poor performance of skin smears WHO adopted a purely clinical operational classification, however the poor specificity of operational classification leads to overdiagnosis and unwarranted overtreatment while the poor sensitivity leads to underdiagnosis of multibacillary (MB) cases with inadequate treatment. Bacilli are more frequently and abundantly demonstrated in tissue sections.

AIMS AND METHODS: We compared WHO classification, slit-skin smears (SSS) and demonstration of bacilli in biopsies (bacterial index of granuloma or BIG) with regards to their efficacy in correctly identifying multibacillary cases. The tests were done on 141 patients and were evaluated for their ability to diagnose true MB leprosy using detailed statistical analysis.

RESULTS: A total of 76 patients were truly MB with either positive smears, BIG positivity or with a typical histology of BB, BL or LL. Amongst these 76 true-MB patients, WHO operational classification correctly identified multibacillary status in 56 (73.68%), and SSS in 43 (56.58%), while BIG correctly identified 65 (85.53%) true-MB cases.

CONCLUSION: BIG was most sensitive and effective of the three methods especially in paucilesional patients. We suggest adding estimation of bacterial index of granuloma in the diagnostic workup of paucilesional patients.