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Early positive culture: what is in a name?

E C Conceicao(1) F Wells(1) M Ismail(1) B Mann(1) J T N Ngom(1) S Omar(1) A Dippenaar(1,2) A Van Rie(2) R Warren(1)

1:South African Medical Research Council Centre for Tuberculosis Research, Division of Molecular Biology and Human Genetics, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa; 2:Department of Family Medicine and Population Health, Global Health Institute, Faculty of Medicine and Health Sciences, University of Antwerp, Antwerp, Belgium

Obtaining quick culture results for Mycobacterium tuberculosis is crucial for effective patient care when utilizing culture-dependent diagnostic assays. The BD BACTEC™ MGIT™ 960 system is a culture method that flags positive at >100 Growth Index (GI), corresponding to 10⁵ to 10⁶ colony-forming units per millilitre. In 1988, Ellner introduced the concept of “early detection” when applying molecular identification methods to mycobacteria growth indicator tubes (MGITs) with GI<100. Recently, culture-based whole-genome sequencing (WGS) for clinical care renewed the interest in primary MGIT cultures to eliminate the sub-culture step. In 2015, Votintseva published a standardized protocol for DNA extraction for WGS from clinical isolates and introduced the term “early positive liquid (MGIT) cultures” (EPC). Since then, many investigators have used EPCs for next-generation sequencing applications. We performed a systematic literature review to map the definitions used for EPC. Of 423 potential studies, 15 were eligible. None of the 15 studies performed DNA extraction immediately after MGITs flagged positive. Most batched isolates, resulting in a combination of true “EPCs” and MGITs with continued growth. Only one study defined the term EPC as “1 to 7 days after positivity and re-incubation”. All other studies failed to report the length of the incubation period between flagging positive and DNA extraction. Because the crucial distinction is between the use of a primary culture versus a sub-culture, we propose to replace EPC with ‘clinical primary MGIT culture’. Future studies should determine the optimal balance between continued growth in MGIT for increased DNA yield and turn-around time.

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