Discordance between Xpert MTB/RIF Assay and Bactec MGIT 960 Culture System for Detection of Rifampin-Resistant Mycobacterium tuberculosis Isolates in a Country with a Low Tuberculosis (TB) Incidence

Abstract

Among 452 samples that were positive by the Xpert MTB/RIF (Xpert) assay and MGIT 960 system (MGIT), 440 and 10 Mycobacterium tuberculosis samples were detected as rifampin susceptible and rifampin resistant, respectively. Two isolates that were rifampin susceptible by the MGIT system were rifampin resistant by the Xpert assay. rpoB sequencing identified a silent (CTG521TTG) mutation in one isolate and a missense (GAC516TAC) mutation in another. The detection of rifampin resistance is imperfect with both the Xpert assay and MGIT system. Any discordant rifampin resistance results should be confirmed by sequencing of the rpoB gene. Multidrug-resistant tuberculosis (MDR-TB) (defined as infection with a Mycobacterium tuberculosis strain resistant to at least the two most effective, rifampin and isoniazid, anti-TB drugs) is prevalent throughout the world, difficult to treat, and associated with higher rates of clinical failure and disease relapse (1, 2). The rapid and accurate laboratory diagnosis of MDR-TB is crucial for effective treatment, which will also limit the transmission of MDR-TB (2, 3). The resistance of M. tuberculosis to rifampin (RMP) in nearly 97% of isolates is due to mutations in an 81-bp rifampin resistance-determining region (RRDR) of the rpoB gene (4). Other RMP-resistant isolates contain mutations in either the N-terminal or cluster II region of the rpoB gene, or the resistance is due to other mechanisms (2, 4). Resistance to RMP is a key determinant in treatment failure and also correlates well with MDR-TB, since Ͼ85% of RMP-resistant M. tuberculosis isolates worldwide are also resistant to isoniazid (INH) (2-4). Molecular assays detect mutations in the RRDR of the rpoB gene for the rapid detection of RMP-resistant M. tuberculosis in clinical specimens and culture isolates (2, 3). The World Health Organization (WHO)-approved tests include two line probe assays, the INNO-LiPA Rif. TB (detecting resistance to RMP only) and the GenoType MTBDRplus (detecting resistance to RMP and INH), as well as the real-time PCR-based automated Xpert MTB/RIF (Xpert) assay (detecting resistance to RMP only) (3, 5). However, these tests are not specific, as silent mutations in the rpoB gene occasionally lead to the detection of false-positive RMP resistance (6-9). The current WHO recommendations are to use the Xpert assay as the initial diagnostic test and start treatment for MDR-TB if an RMP resistance result is expected, or, if unexpected, to repeat Xpert assay testing on another sputum sample, particularly in settings in which the prevalence of RMP-resistant TB is Ͻ15% (10). For those settings, treatment for MDR-TB should be initiated when the Xpert assay repeatedly detects RMP resistance. Treatment should be optimized by following susceptibility testing with other first-line and second-line drugs and confirmatory testing for RMP resistance by phenotypic or other genotypic methods; any discordant RMP susceptibility results can be resolved by sequencing of the rpoB gene (10).Phenotypic drug susceptibility testing ...

Real-Life Clinical Practice of Using the Xpert MTB/RIF Assay in Thailand

“…In other words, none of the samples from the study had false-positive results for rifampicin resistance. However, it should be noted that discordant results between Xpert MTB/RIF and the conventional gold standard DST such as the MGIT has been reported [14,[20][21][22][23]. Discordant results are extremely problematic for physicians in determining the appropriate TB treatment.…”

Teste rápido molecular GeneXpert MTB/RIF para diagnóstico da tuberculose

“…O ensaio ainda pode detectar mutações silenciosas, que não conferem resistência à RIF, sendo também um resultado falso-positivo, o que reforça as recomendações da OMS para a confirmação dos casos de RIF resistente por métodos genotípicos e/ou fenotípicos 14,15 . Somoskovi et al 56 salientaram que o diagnóstico da multirresistência não pode ser baseado em um único teste, mas deve ser sempre estabelecido por uma avaliação e verificação completa de todos os dados clínicos e laboratoriais.…”

“…Dessa forma, a OMS recomenda repetir o teste em outra amostra do paciente. Assim, o tratamento deve ser iniciado após o teste molecular detectar repetidamente a resistência 14,15 .…”

Implementation of the Xpert MTB / Rif Assay in the Kyrgyz Republic, Results, Lessons, Future Prospective

“…В проведенном исследовании, в лаборатории, где проводиться Xpert MTB/RIF и традиционные методы выявления ТБ доставляются разные пробы патологического материла, что может также приводит к несовпадению результатов, общая доля которых составила менее 10% (110/1070). О случаях "ложной" резистентности к РИФ, каковые мы наблюдали в нашем исследовании, также сообщалось в ряде исследований [16,17,18,19]. Также нами выявлено, что 32,7% (36/110) образцов, которые показали, различные результаты были доставлены в лаборатории с задержкой в 4-10 дней.…”