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Comparison of conventional and molecular methods in diagnosis of extrapulmonary (cutaneous) tuberculosis in a tertiary care hospital in Delhi

Hemlata Lall, NP Singh, Meenakshi Chaudhary, Igbal R Kaur.

Abstract
Background: Tuberculosis (TB) is a potentially fatal contagious disease caused by Mycobacterium tuberculosis. The one third of the world’s population is infected with tubercle bacilli. According to World Health Organisation (WHO), global incidence of TB in 2014 was 9 million cases out of which 2.2 million cases were of India only. India has the highest burden of TB. Extrapulmonary Tuberculosis (EPTB) constitutes about 15–20% of TB cases. Cutaneous TB constitutes about 1.5% of all EPTB cases. The incidence of cutaneous TB amongst total dermatology patients varies between 0.1% and 2% in different studies.

Objective: To compare the conventional and molecular methods in diagnosis of extrapulmonary (cutaneous) tuberculosis.

Materials and methods: A cross-sectional study was conducted on consecutive patients of cutaneous tuberculosis in the departments of Microbiology and Dermatology at UCMS & Guru Teg Bahadur Hospital Delhi, from all sample received during the time period between Nov 2010 and March 2012. Conventional method of diagnosis of tuberculosis was performed on biopsy samples i.e. staining and culture methods. The conventional methods of diagnosis are then compared with molecular methods PCR targeting 16S rRNA. The two set of primers were used. The outer (16 SOL,16 SOR) and inner pairs (16 SIL,16 SIR) of primers are expected to be the genus specific and species specific primers for 16S rRNA gene amplification, respectively.

Result: 31 samples received during the time period between November 2010 and March 2012. Female predominance was found in present study 54.8% (17). The clinical types of the received samples from cutaneous tuberculosis patients includes ,TBVC (6.4%), SFD (25.8%) and LV (67.7%). Fifty eight percent of patients were found to be in age group 11–20 years. Ziehl- Neelsen (ZN) staining was performed on smears of all biopsy specimens. The 6.4% (2) were ZN stain positive and 12% (4) were Auramine stain positive and 6 (19.3) were culture positive. Nested PCR was performed on 31 biopsy specimens. Eight (25.8%) specimens were found to be positive for common Mycobacterium species. Out of 8, DNA from 6 biopsy specimens were amplified by both genus specific and species specific primers based on 16S rRNA gene amplification. They were diagnosed as M. tuberculosis infection.

Conclusion: PCR tests offer alternative robust approach to detect M. tuberculosis in paucibacillary EPTB specimens that show rapid results with good diagnostic accuracy. Although, these tests cannot replace the conventional AFB smear, culture identification or histopathological observations but they contribute significantly for an early diagnosis of EPTB and exert an acceptable impact on the clinical management of disease.

Key words: Cutaneous tuberculosis, Ziehl-Neelsen (ZN) stain, Lowenstein Jensen (LJ), nested PCR


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