Utility of CB-NAAT in cutaneous tuberculosis: A prospective observational study

Problem considered

Conventional diagnostic techniques for diagnosing Cutaneous tuberculosis (CTB) have variable sensitivity, specificity and are time-consuming. CB-NAAT is based on real-time PCR and can be helpful in rapid detection of mycobacterial antigens along with the detection of rifampicin resistance. Our study aims at detecting the efficacy of CB-NAAT in the diagnosis of CTB as compared to conventional diagnostic techniques like histopathology and culture.

Methods

Skin biopsies were taken from the representative lesions in 41 patients of CTB. One sample was sent for histopathology whereas the other sample was sent for CB-NAAT, liquid culture -DST (drug sensitivity testing) and smear microscopy.

Results

Out of 41 patients with cutaneous TB, CB-NAAT was positive in 6 (14.63 %), culture was positive in 8 (19.5 %) and histopathology was compatible in 26 (63.4 %) patients. CB-NAAT was positive in 3 out of 8 culture positive cases and 4 out of 26 histopathology compatible cases. The sensitivity and specificity of gene Xpert MTB/RIF in detecting TB when compared to culture were 37.5 % and 91 % respectively and Positive- and Negative-predictive values were 50 % and 85.7 % respectively. Xpert MTB/RIF could identify an additional 3 cases, where all other microbiological investigations were negative. Out of 6 cases that were CB-NAAT positive, 3 showed Rifampicin resistance.

Conclusions

The CB-NAAT test has variable diagnostic accuracy in the diagnosis of CTB which can vary with the clinical specimen. It can prove as a promising tool for early detection of cutaneous TB along with drug-resistant TB especially in the high endemic zones.