Culture positivity of smear negative pulmonary and extrapulmonary tuberculosis- A study from North Kerala, India

INTRODUCTION: The identification of smear negative pulmonary and extrapulmonary tuberculosis continues to remain a diagnostic challenge. This study was conducted in a tertiary care setup in north Kerala to isolate and identify mycobacteria by culture from radiologically and clinically suspected cases of smear negative pulmonary and extrapulmonary tuberculosis. METHODS: A total of 200 samples (100 pulmonary and 100 extrapulmonary) were processed and cultured by automated (MB/BacT) and conventional methods. Heat stable catalase test, nitrate reduction test and detection of MPT 64 antigen were done to aid species identification. RESULTS: Overall culture positivity was 7% (14 isolates - 8 pulmonary and 6 extrapulmonary) of which 92.9% (13) of the isolates were Mycobacterium tuberculosis and 7.1% (1) was Mycobacterium fortuitum (identified by molecular typing). Detection rate by automated method was 7% (14) and by conventional method was only 1.5% (3). CONCLUSION: Despite its shortcomings and low positivity, culture still remains the gold standard for the diagnosis of EPTB and SNPT. However, automated liquid cultures have better isolation rates than the conventional LJ culture. Subjecting these isolates to rapid diagnostic tests like antigen detection and LPA can aid in the early institution of appropriate treatment regimen.