Optimal incubation duration of liquid cultures for assessing culture negative conversion in patients with Mycobacterium avium complex and Mycobacterium abscessus pulmonary diseases.

Purpose: Mycobacterial liquid culturing typically requires six weeks or longer, primarily because of the slow growth rate of Mycobacterium tuberculosis. This study aimed to evaluate the potential of shortening the duration of mycobacterial liquid culturing in healthcare settings with high prevalence rates of non-tuberculous mycobacteria.

Methods: We retrospectively analyzed the relationship between mycobacterial species and time to positive testing of liquid cultures from sputum samples using the Mycobacteria Growth Indicator Tube system over a 3.5-year period beginning in July 2020 at a university hospital in Japan.

Results: We analyzed 15,147 sputum culture samples and found a 1.1% positivity rate for Mycobacterium tuberculosis complex, while the rates for Mycobacterium avium complex and Mycobacterium abscessus were 17.6% and 2.1%, respectively. The median time to positivity was 17 days for Mycobacterium tuberculosis complex, 9 days for Mycobacterium avium complex, and 4 days for Mycobacterium abscessus. Comparing a 4-week culture period with an eight-week period, the positivity rates for Mycobacterium avium complex and Mycobacterium abscessus were 97.0% and 99.4%, respectively.

Conclusion: In settings with a high incidence of non-tuberculous mycobacteria, the basic liquid culturing period can be safely shortened to 4 weeks without significantly compromising detection sensitivity, except for the samples that are highly suspected to contain tuberculosis, extremely slow-growing mycobacteria, smear-positive, or nucleic acid amplification testing positive.