Xpert MTB/XDR is a nested real-time polymerase chain reaction-based test used to detect resistance to isoniazid, fluoroquinolones, amikacin, kanamycin,
Capreomycin and ethionamide. It detects fluoroquinolone resistance through the amplification of gyrA and gyrB genes.
1 A recent Cochrane review estimated the sensitivity and specificity for Xpert MTB/XDR to detect fluoroquinolone resistance in sputum samples comparison to phenotypic DST to be 93.2% (ranging from 88.1-96.2%) and 98.0% (ranging from 90.8-99.6%).
2 Despite the high sensitivity, our patient was falsely reported negative for fluoroquinolone resistance. One possible reason for such a discrepancy could be due to improper sample handling, storage and other technical errors which may lead to a reduction in the number of organisms present in the sample, thus affecting detection of MTB deoxyribonucleic acid.
1 This is unlikely to be the case in our patient as the MTB-load detected for the same sample on Xpert MTB/Rif was medium.
Another possible explanation for this result lies in the type of sample tested. Unlike sputum, tissue samples have not been evaluated for testing by the Xpert MTB/XDR panel. In addition, blood and white blood cells are considered as “potentially-interfering substances” for the Xpert MTB/XDR test and their presence in the lymph node sample may result in aberrant results.
1 Interference by organisms can also affect the reporting of fluoroquinolone resistance by Xpert MTB/XDR. The ATCC 0927 strain of M.marinum at concentrations greater than 104 colony forming units (CFU)/mL can interfere with the gyrA signal and thus leads to the suppression of one or more melting temperature peaks. However, this would lead to the reporting of a fluoroquinolone indeterminate call.
1,3
The phenomenon of heteroresistance may also explain these discordant results. If the tissue sample contained a mixture of fluoroquinolone sensitive and resistant strains, Xpert MTB/XDR may not be able to pick up the resistant strain, if the resistant strain was present at levels below the limit of detection (LoD). Cao et al.
3 found that mixtures of populations with A90V and S91P gyrA mutations were not reliably reported to have low fluoroquinolone resistance by the Xpert MTB/XDR panel. Rather, they were either reported as fluoroquinolone resistant or sensitive by the assay. They also found that reporting of gyrB mutations in mixed samples as fluoroquinolone resistant, required a prevalence of 60% of the mutant strains.
3 However, heteroresistance is also an unlikely cause for the discrepancy in our patient as the estimated LoD of Xpert MTB/XDR (71.9 CFU/mL for MTB detection and 95.5 CFU/mL for fluoroquinolone susceptibility detection) is much lower than that of LPA (~10,000 CFU/mL).
3,4
Lastly, the presence of mutations or polymorphisms in probe bindings regions or the primer region may result in an inability to be detected by the Xpert MTB/XDR panel and result in a falsely drug-sensitive result. Cao et al.
3 reported that while the assay was vigorous in detecting gyrA mutants, it missed detecting one gyrB mutant (C5389A), and this could lead to the reporting of a false-sensitive result.
Ultimately, since the World Health Organization considers phenotypic DST on liquid media such as the MGIT system as the reference standard and finds fluoroquinolone resistance detection by this method to be reliable, we treated our patient in concordance with the DST results as preXDR-TB.
5| References : |
- Cepheid. Xpert® MTB/XDR. Cepheid. 2021 Apr. Available from: https://www.cepheid.com/content/dam/www-cepheid-com/documents/package-insert-files/Xpert%20MTB-XDR%20ENGLISH%20Package%20Insert%20302-3514%20Rev%20C.pdf [Accessed on 18th January, 2025].
- Pillay S, Steingart KR, Davies GR et al. Xpert MTB/XDR for detection of pulmonary tuberculosis and resistance to isoniazid, fluoroquinolones, ethionamide, and amikacin. Cochrane Database Syst Rev. 2022 May 18;5(5):CD014841.
- Cao Y, Parmar H, Gaur RL et al. Xpert MTB/XDR: a 10-Color Reflex Assay Suitable for Point-of-Care Settings To Detect Isoniazid, Fluoroquinolone, and Second-Line-Injectable-Drug Resistance Directly from Mycobacterium tuberculosis-Positive Sputum. J Clin Microbiol. 2021 Feb 18;59(3):e02314-20.
- Ninan MM, Gowri M, Christopher DJ et al. The diagnostic utility of line probe assays for multidrug-resistant tuberculosis. Pathog Glob Health. 2016 Jun-Jul;110(4-5):194-9.
- 2.1 Conventional diagnostic tests for the diagnosis of TB | TB Knowledge Sharing. Available from: https://tbksp.who.int/en/node/734 [Accessed on 18th January, 2025].
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