‘Antimicrobial resistance’ is a useful term, but the readership of this newsletter does not need reminding that it is a broad term that somewhat oversimplifies the science.

Clinically, ‘breakpoints’ are used to define a minimum inhibitory concentration above which a pathogen is susceptible to a given drug. However, this too is an over-simplification, because breakpoints are influenced by factors such as carriage and expression of resistance genes, the site of infection, the pharmacokinetics of the drug and so on. Nor is a pathogen simply ‘resistant’ or ‘sensitive’, as this depends upon the dose of antibiotic to which it is exposed.

The clinical breakpoint is used to guide treatment, and is not a fixed characteristic of either drug nor pathogen. For this reason, regular reviews are important to ensure that breakpoint values are kept up to date. The World Health Organisation (WHO) has recently published a technical report based on a systematic review performed by Dr Koser and colleagues to update the current advice on the use of antibiotics to treat tuberculosis. Twenty breakpoints have been revised, including to fluoroquinolones and to second-line agents and add-ons such as bedaquiline. The report considers the response of different clinical isolates to treatment, as well as the impact of the chosen methodology for culturing the bacteria, such as 7H11 media vs Bactec mycobacterial growth indicator tube (MGIT) assay.