Introduction: Leprosy is a chronic granulomatous disease caused by Mycobacterium leprae, diagnosed based on clinical signs, histopathological findings and demonstration of acid fast bacilli on skin biopsy. But diagnosis becomes challenging in case of tuberculoid spectrum of leprosy where diagnosis is made on histological findings
like nerve destruction and granuloma. The histological findings like granuloma may simulate other granulomatous dermatosis like sarcoidosis. In such cases immunostaining with S-100 helps in differentiating leprosy from other granulomatous dermatosis on the basis of nerve staining. Materials and Method: This is a prospective descriptive study on 50 skin biopsy specimen diagnosed as leprosy
and other cutaneous granulomatous dermatosis. Histological examination was done using H&E and fite faracco. Immunohistological examination was done after staining with S-100. Diagnostic efficacy of test in terms of sensitivity were calculated using histopathological features as gold standard. Correlation between pattern of neutritis by S-100 staining and agreement between histological diagnosis were analysed. Result: Out of total 50 skin biopsies, 30 were diagnosed as leprosy and 20 cases were other granulomatous dermatosis. Maximum cases among leprosy were of tuberculoid leprosy (46.67%). Sensitivity of S-100 in nerve identification in tuberculoid leprosy was 100% whereas it was 85.71% with H&E. Fragmented (64.29%) and infiltrated (21.43%) pattern was most common in tuberculoid spectrum of leprosy whereas lepromatous spectra showed mostly fragmented pattern (80%). Intact nerves were seen in all the other cutaneous granulomatous lesions. Conclusion: Therefore, S-100 is an effective auxillary tool in early diagnosis and differentiation of leprosy from other granulomatous dermatosis.

Keywords: Granulomatous dermatosis; Leprosy; Nerve staining; S-100 immunostaining.