Drug susceptibility testing

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.