The contribution of eight-week incubation of solid media used alongside liquid culture media in tuberculosis diagnosis

It is recommended to use both solid and liquid culture media together for the rapid and reliable diagnosis of tuberculosis. However, to report negative culture results, liquid media should be incubated for six weeks, and solid media for eight weeks. Current guidelines do not include recommendations on whether a shorter incubation period for solid media could suffice when both media are used together. In our study, we aimed to investigate the diagnostic benefit of extending the incubation of solid media by two additional weeks (after the sixth week, when the liquid media resulted as negative) when used in conjunction with a liquid media-based automated culture system. We reviewed data from 18,901 samples cultured using the Bactec MGIT 960 automated culture system and Löwenstein-Jensen (LJ) medium between 01.01.2011 and 31.12.2021, in the mycobacteriology laboratory. Statistical analyses of the growth times of samples in which growth was detected in at least one of the two culture media were performed and the contribution of incubation of solid media for more than six weeks to the diagnosis was calculated. Among 1,130 samples that tested positive for mycobacteria in at least one medium, growth times were available for 1,110 samples [846 were Mycobacterium tuberculosis complex (MTBC), and 264 were non-tuberculous mycobacteria (TDM)]. Among 1110 samples, out of 47 (%4.23) samples in which mycobacteria growth was detected in LJ medium between 6-8 weeks, no growth was detected in MGIT in six (%0.54). Among MTBC positive samples, out of 36 (%4.25) samples in which growth was detected in LJ medium between 6-8 weeks, no growth was detected in MGIT in four (%0.47). Among TDM positive samples, out of 11 (%4.17) samples in which growth was detected in LJ medium between 6-8 weeks, no growth was detected in MGIT in two (%0.76). The diagnostic contribution of incubating LJ media beyond six weeks when used alongside liquid culture systems was found to be only 0.47% for tuberculosis diagnosis. It was concluded that reporting both liquid and solid media as negative at the end of six weeks could significantly benefit patients, clinicians, and laboratories by enabling earlier reporting protocols.