The importance of multiple samples in mycobacterial recovery: A 10-year retrospective study

Instead of the three samples previously recommended, the World Health Organization considered that two sputum samples are sufficient for the diagnosis of pulmonary tuberculosis. Our goal is to evaluate the value of multiple samples in the microscopic and cultural examination of Mycobacterium tuberculosis complex (MTBc) and nontuberculous mycobacteria (NTM).

Through a 10-year retrospective study, including 38,887 sputum samples from 11,676 patients, the relative contribution of each sputum specimen for acid-fast bacilli detection and mycobacteria isolation was determined by calculating the yield of the first sample and the incremental yield of the second, third, and more than three samples.

With regard to patients with MTBc isolation, we found that 95.2% (435/457) of the cases could be detected in the first and second smear and 97.9% (659/673) in the first and second culture, with a very low increment of the third sample, corresponding to 3.1% (14/457) in smear and 1.3% (9/673) in culture. Among patients with an NTM strain, the percentage of cases detected in the first and second smear and culture was 69% (20/29) and 86.5% (316/365), respectively. In these patients, we found an increment yield of the third sample of 20.7% (6/29) for smear and 10.5% (38/365) for culture. The incremental yield of third and above-third sample for NTM detection was of 31% (9/29) in smear and 13.5% (49 of 365) in culture.

We found an added value of multiple samples in NTM recovery. However, our findings suggest that only two samples are required for the diagnosis of pulmonary tuberculosis.