Turnaround time of whole genome sequencing for mycobacterial identification and drug susceptibility testing in routine practice

Objectives: Until recently whole genome sequencing (WGS) for mycobacteria has been restricted mostly to the research setting. However, in 2017 Public Health England has implemented WGS for routine mycobacterial identification and susceptibility testing for Mycobacterium tuberculosis. Our objective was to evaluate the impact of this change on the laboratory turnaround times and availability of results.

Methods: Over the years 2016 and 2017, the period 1 January to 30 April was selected to represent before and after implementation of WGS. Prior to 2017, line probe assays were used for mycobacterial species identification. Turnaround times for the different steps of the diagnostic process were evaluated for all positive mycobacterial cultures that were sent from our hospital to the Reference Laboratory during the study period.

Results: A total of 161 positive mycobacterial cultures were sent to the Reference Laboratory. Half of the isolates (n=81/161, 50%) were M. tuberculosis and 80/161 (50%) were non-tuberculous mycobacteria. The median number of workdays for mycobacterial species identification was 1 day (interquartile range (IQR) 1-3) in 2016 and 6 days (IQR 5-7) in 2017, p <0.001. For M. tuberculosis complex, the median time to drug susceptibility testing results, either molecular or phenotypic, was 12 days (IQR 11-18) in 2016 and 8 days (IQR 7-10) in 2017, p <0.001.

Conclusions: Routine WGS performed well in this setting for mycobacterial identification and susceptibility testing for M. tuberculosis and decreased time to drug susceptibility testing results. There was an increase in turnaround times for species identification using WGS, when compared with the previous methods.