Abstract
Background Universal access to drug susceptibility testing is key to ending TB. UKMYC5 is a 96-well microtitre plate designed by the Comprehensive Resistance Prediction for Tuberculosis: an International Consortium (CRyPTIC) which has potential to determine, at low cost, the MICs for 14 different anti-TB drugs, including several new and repurposed compounds. It is a dry-format plate and therefore easy to transport and store.
Objectives Determine how long to incubate the plates before reading, and the optimal reading method. Establish the reproducibility of the UKMYC5 plate and compare it to established methods.
Methods UKMYC5 plates were tested by seven laboratories on four continents using a panel of 19 external quality assessment (EQA) strains, including H37Rv. MICs were measured from each plate by two readers using three methods (mirrored-box, microscope and Vizion™ Digital viewing system) at four different timepoints. All EQA strains were whole-genome sequenced and phenotypically characterized by MGIT960, 7H10/7H11 agar and resazurin microtitre assay.
Results The optimum duration to incubate a plate is 14 days. The within-and between-laboratory reproducibilities for the best performing methods (mirrored-box and Vizion) were both 95% and 92%, respectively. One site was identified as requiring re-training and one drug (para-aminosalicylic acid) produced inconsistent results.
Conclusions MICs measured using the UKMYC5 microtitre plate (i) are reproducible, (ii) compare well with the results of several established methods and (iii) correlate with the presence or absence of genetic mutations that confer resistance. This study provides the evidence that this assay can be deployed by TB reference laboratories world-wide as a diagnostic and research tool.