Why are these terms not yet recognised by WHO?
Terms such as “totally drug resistant” have not been clearly defined for tuberculosis. While the concept of “total drug resistance” is easily understood in general terms, in practice, in vitro drug susceptibility testing (DST) is technically challenging and limitations on the use of results remain: conventional DST for the drugs that define MDR and XDR-TB has been thoroughly studied and consensus reached on appropriate methods, critical drug concentrations that define resistance, and reliability and reproducibility of testing.[9] Data on the reproducibility and reliability of DST for the remaining SLDs are either much more limited or have not been established, or the methodology for testing does not exist. Most importantly, correlation of DST results with clinical response to treatment has not yet been adequately established. Thus, a strain of TB with in vitro DST results showing resistance could in fact, in the patient, be susceptible to these drugs. The prognostic relevance of in vitro resistance to drugs without an internationally accepted and standardised drug susceptibility test therefore remains unclear and current WHO recommendations advise against the use of these results to guide treatment.[10]
Lastly, new drugs are under development, and their effectiveness against these “totally drug resistant” strains has not yet been reported.