Comparative study of PCR, smear examination and culture for diagnosis of cutaneous tuberculosis

Diagnosis of tuberculosis (TB), especially cutaneous TB by conventional laboratory method is unreliable and time consuming. We assessed the utility of Polymerase Chain Reaction (PCR) test vis a vis other laboratory tests in 37 clinical samples of skin biopsy from equal number of patients with different variants of cutaneous TB. The PCR test amplifying 165bp region of 65kDa antigen coding gene specific for M. tuberculosis was performed on skin biopsy samples obtained from cases with a strong clinical evidence of cutaneous TB. The samples were also subjected to other laboratory tests e.g. smear examination, conventional (LJ based culture) and rapid BACTEC culture and histopathological examination for mycobacteria. Significant difference (p<0.05) was observed in the sensitivity of PCR test vis-a-vis other tests e.g. smear examination, LJ and BACTEC culture. PCR test showed a higher sensitivity than histopathological examination but the difference was not found to be statistically significant (p>0.05). PCR test showed the maximum positivity of 79.4% followed by histopathology (73.5%), BACTEC culture (47.5%), LJ media culture (29.4%) and smear examination (5.8%). The sensitivity and specificity of PCR test employing culture as the "gold standard" were 95.2% and 100%. The mean time taken for a positive result in different tests were less than 24 hours for smear examination, 1 day for PCR test, 23.42 days for BACTEC culture and 38.02 days for LJ culture. These results show that PCR amplification of 165bp region of 65kDa antigen coding gene of M. tuberculosis is a rapid and sensitive test for diagnosis of cutaneous TB using skin biopsy samples.