4.2.4
Drug susceptibility testing
National TB Management Guidelines, 2018 Edition. Chapter 4, Approach to TB diagnosis.
Clinical description
DST is required to make a definitive diagnosis of MDR- and XDR-TB. It can be done by using either phenotypic assays (e.g., conventional proportion method) or genotypic techniques (e.g., the line probe assay (LPA) or Xpert MTB/RIF) techniques.
- Phenotypic DST: can be performed by either observation of growth or by metabolic
inhibition of the bacilli in a culture medium containing anti-TB drugs. It is considered the gold standard technique to test susceptibility to various TB drugs. However, the technique can only be performed on MTB isolates grown on culture media.
- Molecular DST techniques: are DNA PCR technologies that are specifically designed
to detect/confirm genetic mutations associated with resistance to TB drug. They can produce rapid results. Their role is limited to diagnostic/screening purpose and cannot be used to monitor treatment response as they do not distinguish between live and dead bacilli.
At present, Xpert MTB/RIF and LPA are the two (genotypic) techniques recommended in Malawi.
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