Sensitivity and specificity of methods of classification of leprosy without use of skin-smear examination

A 12-month cohort of 2664 new leprosy cases in Bangladesh has been analyzed to provide information about the sensitivity and specificity of two different methods of classifying leprosy into paucibacillary (PB) and multibacillary (MB), if the results of skin-smear examination are not taken into account. The two methods are: 1) a procedure based on counting skin lesions recommended by the World Health Organization (WHO) (<6 skin lesions = PB, ≥6 skin lesions = MB); and 2) the 'Bangladesh method' (<10 skin and nerve lesions = PB, ≥10 skin and nerve lesions = MB). In the latter system, any degree of nerve enlargement is taken to be a nerve lesion. The WHO method was found to be 89% sensitive and 88% specific at detecting smear-positive MB cases from among the cohort; the Bangladesh system, 92% sensitive and 88.6% specific. The WHO method did not detect 18 smear-positive cases as MN; the Bangladesh method left 13 smear-positive cases unclassified as MB. Several of these 'missed' (false-negative) cases had a high bacterial index. The WHO system of classifying leprosy cases as MB is simple to apply and has a reasonable balance between sensitivity and specificity. However, it must be recognized that the system will lead to a small but significant number of skin-smear-positive MB cases being treated with a PB treatment regimen.