Tuberculosis in Adults and Children by Dorothee Heemskerk Maxine Caws Ben Marais & Jeremy Farrar

Author:Dorothee Heemskerk, Maxine Caws, Ben Marais & Jeremy Farrar
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

4.4 Diagnosing Drug-Resistant Tuberculosis

A major impact of the scale-up of Xpert MTB/RIF is increased detection of RIF resistance, which is a surrogate marker for MDR TB. Classical diagnosis of drug resistance in M. tuberculosis involves culture of the bacilli on solid or liquid media and comparison of growth between drug-free and drug containing media. Even with the advent of direct liquid culture methodology, detection of drug resistance takes over two weeks, and with indirect methods, two months or more. Standardisation of drug susceptibility testing for the antituberculous drugs is difficult and should be performed in a biosafety level 3 laboratory by trained personnel participating in an external quality assurance scheme. Isoniazid, rifampicin and streptomycin are the most reliable drug susceptibility tests.

Molecular detection of drug resistance mutations provides a rapid alternative, but the accuracy of these tests varies according to the drug. Rifampicin resistance detection is the most accurate, as 95 % of phenotypically rifampicin resistant strains carry a mutation in the 81 base pair rifampicin—resistance-determining-region (RRDR) of the rpoB gene. For isoniazid, molecular methods can detect approximately 75 % of phenotypically resistant strains by detecting mutations in the katG gene or InhA promoter region. Development of commercial NAAT for the other antituberculous drugs has been hampered by incomplete understanding of the molecular mechanisms of resistance. The principal commercial NAAT for drug resistance are the Xpert MTB/RIF test and the line probe assays . The most recent pooled estimates for M. tuberculosis detection by Xpert MTB/RIF were sensitivity of 88 % (95 %CI; 83–92%) and specificity of 98 % (95 %CI; 97–99 %); for rifampicin resistance sensitivity 94 % (95 %CI; 87–97 %) and specificity 98 % (95 %CI; 97–99 %) (Steingart et al. 2014). Line Probe assays detect both rifampicin and isoniazid resistance simultaneously and the MTBDR-sl assay detects resistance to fluoroquinolones, ethambutol and aminoglycosides. The MTBDR-sl test has high sensitivity for fluoroquinolones, but low sensitivity for aminoglycoside and ethambutol resistance (Feng et al. 2013). However, specificity is high for all drugs and therefore the test can be used to detect resistance but should not be used to rule-out resistance. Unfortunately, the need remains to confirm susceptibility by laborious phenotypic DST. Rapid sequencing techniques provide more comprehensive drug susceptibility data but are not yet widely available beyond research settings.

Several non-commercial phenotypic DST approaches have been developed including microscopic observation drug susceptibility testing (MODS), nitrate reductase assay (NRA) and colorimetric redox indicator (CRI) tests. A MODS test kit is now available to improve standardization (Hardy diagnostics). In 2010 WHO issued a recommendation that MODS could be used as an ‘interim’ approach for increased DST in high-burden countries but concluded that there was insufficient data to recommend NRA or CRI. Reservations particularly around biosafety and quality control have limited scale-up of the techniques (Image 4.1).

Image 4.1Mycobacterial cording in MODS. Image courtesy of Dr. Dang Thi Minh Ha

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