Rapid Molecular Assays for the Diagnosis of Drug-Resistant Tuberculosis.

The recognition that drug-resistant tuberculosis (DR-TB) poses a major threat to global tuberculosis (TB) control efforts has catalysed the development of new and urgently needed TB diagnostics. The full beneficial impact of the subsequent flood of new TB diagnostic tests into the market can only be realised if these diagnostic tests are readily accessible to TB programs and contribute to improved patient outcomes. Although phenotypic drug-susceptibility testing remains the gold standard, an improved understanding of the relationship between mutations and different levels of drug resistance coupled with the advantages of molecular diagnostics could result in rapid molecular diagnostic tests replacing phenotypic drug-susceptibility testing. Successful diagnostics need to diagnose all forms of drug-resistant TB prevalent in each geographic region. Given the finite number and often limited availability of effective drugs for DR-TB, the diagnostic test must be able to detect all clinically important types of resistance to available anti-TB drugs. However, less comprehensive resistance profiling may be sufficient in settings where extensively drug-resistant TB (XDR-TB) and pre-XDR are absent. Rapid molecular diagnostic tests for DR-TB detection suitable for DR-TB endemic settings should be accurate, inexpensive, suitable to be performed on an easily accessible sample, detect prevalent circulating drug-resistant strains, and provide results within a short turnaround time to enable timely treatment initiation. In this review, we appraise the wide range of molecular diagnostics for DR-TB endorsed by the World Health Organisation, discuss the challenges in the development and rollout of rapid molecular DR-TB tests in low- and middle-income countries, and highlight user perspectives and cost-effectiveness factors that influence their utility.