Discordance across Several Methods for Drug Susceptibility Testing of Drug-Resistant Mycobacterium tuberculosis Isolates in a Single Laboratory

Abstract

bGiven the increases in drug-resistant tuberculosis, laboratory capacities for drug susceptibility testing are being scaled up worldwide. A laboratory must decide among several endorsed methodologies. We evaluated 87 Mycobacterium tuberculosis isolates for concordance of susceptibility results across six methods: the L-J proportion method, MGIT 960 SIRE AST, Gene/Xpert MTB/ RIF, GenoType MTBDRplus line probe assay, MycoTB MIC plate, and a laboratory-developed mycobacteriophage quantitative PCR (qPCR)-based method. Most (80%) isolates were multidrug resistant. Of the culture-based methods, the mycobacteriophage qPCR method was fastest, the L-J proportion method was the slowest, and the MGIT method required the most repeat testing (P < 0.05). For isoniazid (INH), 82% of isolates were susceptible by all methods or resistant by all methods, whereas for rifampin (RIF), ethambutol (EMB), and streptomycin (STR), such complete concordance was observed in 77%, 50%, and 51% of isolates, respectively (P < 0. G iven the increasing rates of multidrug-resistant tuberculosis (MDR-TB) isolates worldwide and the emergence of extensively drug-resistant TB, the development of rapid and accurate methods for drug susceptibility testing (DST) of Mycobacterium tuberculosis isolates is a global priority. The culture-based proportion method that employs Löwenstein-Jensen medium is a World Health Organization (WHO)-recommended method that has been in wide use for over 50 years (1, 2). Such solid medium-based DST methods are slow, requiring readings at 4 to 6 weeks, which delays the detection of drug resistance and risks inappropriate treatment and spread of drug-resistant strains. This deficiency has led to the development of newer DST methodologies, including liquid culture systems and molecular line probe assays, which have also received recommendations from WHO (3, 4).As a consequence, laboratories have seen an accumulation of methods from which they must choose, and a given specimen or isolate may be tested across a variety of methods. A natural consequence is that discrepancies between methods may be encountered. Such discrepancies may be of little consequence in certain scenarios, such as streptomycin (STR) resistance in settings that use primarily isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB), but they may have critical implications for treating MDR-TB in areas where the arsenal of drugs is limited in number and potency. Discordance is becoming a vexing aspect for TB clinicians and will likely increase in frequency as new methodologies are adopted, yet its extent has received little attention. Most diagnostic evaluations examine one new method against one gold standard reference method, not several methods against each other. Additionally, most diagnostic evaluations are performed on predominantly drug-susceptible isolates, often highly susceptible clinical isolates or reference strains, and thus discrepancies between methods would be expected to be rare.For this reason, we prospectively examined 87 mos...

A Comparison of the Sensititre MycoTB Plate, the Bactec MGIT 960, and a Microarray-Based Molecular Assay for the Detection of Drug Resistance in Clinical Mycobacterium tuberculosis Isolates in Moscow, Russia

“…Unexpectedly, we obtained a significantly higher correlation between MGIT and MycoTB with EMB (0.79) compared to the previously published value of 0.30 [43]. Generally, the presence of mutations in the embB 296–497 codon region increases the resistance of the strain to EMB to varying degrees, which might be lower than the plasma drug level of 5 μg/ml [44].…”

Diagnostic Accuracy of a Molecular Drug Susceptibility Testing Method for the Antituberculosis Drug Ethambutol: a Systematic Review and Meta-Analysis

“…It has been reported that the results of the Bactec 460 method and AgarPM were discordant (54,55). It was reported that false susceptibility to EMB is of little consequence in settings of susceptible M. tuberculosis isolates (55). Thus, the effect of different drug concentrations on the evaluation of molecular detection of EMB resistance was analyzed.…”

Assessment of a 96-Well Plate Assay of Quantitative Drug Susceptibility Testing for Mycobacterium Tuberculosis Complex in China

“…One study reported that there were a number of ofloxacin-susceptible strains by conventional methods but resistant by MYCOTB and genotypic methods ( gyrA mutant) [3]. Another study conducted by Sayera et al showed that DST of streptomycin worked well with MYCOTB but less well for the conventional phenotypic methods[4] . …”