Performance of the low-cost phenotypic thin-layer agar MDR/XDR-TB Colour Test (first generation, 1G, Color Plate Test) for identifying drug-resistant Mycobacterium tuberculosis isolates in a resource-limited setting

Background: The accessible, easy to use and timely, diagnosis of tuberculosis (TB) drug-susceptibility, including multi-drug resistant (MDR-) TB and extensively-drug resistant (XDR-)TB is often challenging, particularly in resource-constrained settings. We therefore evaluated the phenotypic thin-layer agar based MDR/XDR-TB Colour Test, which is also referred to as the “First Generation (1G) Color Plate Test (TB-CX)” performance for detecting resistance of Mycobacterium tuberculosis ( Mtb ) isolates to selected anti-TB drugs versus other tests routinely used in our setting. Methods: A cross-sectional study was conducted on Mtb clinical isolates stored at the Armauer Hansen Research Institute TB laboratory in Addis Ababa, Ethiopia. Drug-susceptibility testing was performed on 78 Mtb isolates for isoniazid, rifampicin, and moxifloxacin using the Colour Test and the Indirect Proportional Method (IPM) “in house” assay. Isoniazid and rifampicin were also evaluated by the Mycobacterial Growth Indicator Tube (MGIT) commercially available assay. Test accuracy was calculated as % agreement with 95% confidence intervals (95%CI). Results: The median (range) times in days determining Mtb resistance or susceptibility for the Colour Test, IPM and MGIT assays were of 9 (5–18), 15 (13–18) and 18 (14–21) days, respectively. The Colour Test provided results significantly (p < 0.001) more rapidly than the IPM or MGIT assays. Colour Test accuracy compared to MGIT DST for detecting isoniazid and rifampicin resistance and MDR-TB was 88% (95%CI = 81–96), 92% (95%CI = 86–98), and 94% (95%CI = 88–99), respectively. Colour Test accuracy compared to IPM to detect isoniazid, rifampicin resistance and MDR-TB was 92% (95%CI = 86–98), 81% (95%CI = 72–90), and 90% (95%CI = 83–96). IPM test accuracy compared to MGIT DST for detecting isoniazid and rifampicin resistance and MDR-TB was 91% (95%CI = 85–97), 83% (95%CI = 75–92), and 85% (95%CI = 77–93), respectively. Moxifloxacin drug-susceptibility testing could not be assessed because only two isolates showed evidence of resistance. Conclusion: The accuracy of Mtb drug-susceptibility testing was similar comparing: Colour Test versus IPM, Colour Test versus MGIT; and comparing IPM versus MGIT. The Colour Test was easy to use and determined drug-susceptibility significantly more rapidly than the IPM and MGIT assays. Thus, implementing the Colour Test in clinical settings could make drug-susceptibility testing more accessible and rapid in high TB burden, and resource-constrained settings, including in Ethiopia.