4.1 How is AMR measured?

Other modules have introduced the process of testing bacteria for antimicrobial resistance. The process involves isolating the target bacteria and conducting susceptibility testing for one or more antimicrobials. Depending on the type of testing performed, the results might be reported as the minimum inhibitory concentration (MIC), or as the resistance category (e.g. susceptible, intermediate or resistant).

Across human and animal health there are three broad indicators for AMR:

• Proportion of specimens from which a target pathogen was isolated: This is usually reported for specimens from livestock and aquatic animals, as sampling is often performed on healthy animals that are about to enter the food chain. Target pathogens are usually zoonotic pathogens (disease-causing bacteria that can be transmitted from animals to people) or, if looking for possible reservoirs of resistance genes, might also focus on commensal bacteria (bacteria that are naturally present but do not usually cause disease). In human health it is often reported for screening programmes, for example MRSA screening.
• Proportion of isolates that were resistant to each antimicrobial: This is a commonly used indicator in livestock, aquatic and human AMR surveillance.
• Minimum inhibitory concentration: Results from broth/agar dilution testing are often presented as distributions of MICs for particular isolates and antimicrobials, which can be used to develop breakpoints to distinguish between resistant and wild-type organisms.

These indicators are often categorised by the population level and type. AMR may be measured and reported at a specific site such as a hospital, or across all hospitals in the country. Results are also often reported according to the specific population to which they apply. For example, in a livestock study the proportion of resistant isolates from commercial broilers may be presented, but you can’t assume that the results would be the same for backyard poultry. In public health, a distinction is made between patients staying in hospital compared to patients in the community.

The Global Antimicrobial Resistance Surveillance System (GLASS) is the main global surveillance system for AMR in human health. GLASS reports [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] provide an example of how AMR is measured at global level. You can also refer to the module Introducing AMR surveillance systems for more information.