Incubation time of mycobacterial cultures: how long is long enough to issue a final negative report to the clinician?

Traditionally, solid culture media for mycobacteria are kept for up to 8 weeks before a negative result is reported to the physician. Even though it is well documented that liquid media detect mycobacteria much earlier, most incubation protocols still require a maximum of 6 weeks (2). In Switzerland, it is common practice to comply with these protocols, and no reports to the clinicians are currently generated prior to 6 to 8 weeks. Major recent changes in the Swiss health system, together with increasing economic pressure to bill patients as early as possible, prompted us to study whether mycobacterial cultures may be considered negative sooner. In our academic teaching hospital, comprising nearly 1,000 beds, we have evaluated the time to detection (TTD) of mycobacteria in culture over an 8-month period. There were 843 clinical specimens inoculated to the standard culture media (MGIT, i.e., the Bactec MGIT 960 system [Becton Dickinson {BD} Microbiology Systems, Sparks, MD], Lowenstein-Jensen [LJ; homemade], and Middlebrook 7H10/sel7H11 agar [BD]) on a biplate containing the classical 7H10 medium on one half of the plate and the 7H11 medium (supplemented with polymyxin B, carbenicillin, amphotericin B, and trimethoprim lactate) (Becton Dickinson) on the other half. Acid-fast bacilli (AFB) were detectable in 31 clinical specimens by smear, culture, and optionally, direct molecular tests. Seven specimens were excluded because either they originated from tuberculosis (TB) patients under treatment (smear positive/culture negative) or there was a high clinical suspicion of tuberculosis (culture negative/confirmed by strand displacement amplification [SDA; BD]) (3). Seven of the 24 specimens containing AFB were smear positive. Mycobacterium tuberculosis grew in 13 specimens, Mycobacterium bovis grew in one specimen, and nontuberculous mycobacteria (NTM) grew in 10 specimens (Mycobacterium avium, n 1; Mycobacterium chelonae, n 3; Mycobacterium fortuitum, n 1; Mycobacterium gordonae, n 1; Mycobacterium malmoense, n 1; Mycobacterium peregrinum, n 1; Mycobacterium simiae, n 2). Mycobacteria were identified by either GenoType line probe assays (Hain Lifescience, Nehren, Germany) (1) or in-house 16S rRNA gene sequencing (4). With the exception of 4 clinical specimens out of 24 AFBpositive specimens, the broth medium expectedly allowed the most rapid growth. Irrespective of smear results, the mean time to detection (TTD) in the MGIT broth was 15.3 days for all mycobacteria, 17.1 days for the M. tuberculosis complex, and 12.6 days for NTM, while one MGIT result remained negative. On LJ, the mean TTD was 23.9 days for all mycobacteria, 25.4 days for the M.