2.2 Which set of standards should you use?
CLSI and EUCAST use similar methodologies and criteria to set the reference standards for AST, but there are some differences between the two (Cusack et al., 2019), such as the type of agar used for certain bacteria. Both are recognised by the World Health Organization’s Global Antimicrobial Resistance Surveillance System (GLASS) and the World Organisation for Animal Health (OIE). However, EUCAST guidelines are developed as part of an open and public consultation, are freely available, and are more widely used outside of North America (WHO, 2020).
CLSI and EUCAST guidelines are used to produce antimicrobial susceptibility data from clinical samples that can be analysed at both a local and a national level. The interpretation of these results may then be used to inform animal and public health policy recommendations, and also as part of a One Health approach to AMR surveillance. One Health is a concept that recognises the interdependency of animal health and human health, and the ecosystems in which they evolve.
CLSI and EUCAST guidelines are used to guide treatment decisions by veterinarians and clinicians.
Are the same bacterial species important in animal and human diseases?
While there are bacterial pathogens common to animals and humans, there are also bacterial species that are important in animal disease that are not important in human disease and vice versa.
The same antibiotic may be used to treat bacterial disease in animals and humans, but separate reference standards should be used to determine treatments in veterinary and human medicine since animals and people may metabolise the same drug differently.
‘Human’ reference standards are published by both systems but only CLSI (CLSI VET) currently provides guidelines and standards specifically for use in veterinary medicine and research. EUCAST VET guidelines and standards are still in development (EUCAST, 2020a). When results are used to inform veterinarians’ clinical decisions, it is important to use veterinary guidelines. When the results of tests for isolates of animal origin are to be compared with human health results in a One Health approach to AMR surveillance, the same human reference standards should be used for all tests so that valid comparisons can be made.
Activity 2: Using reference standards
Use the space below to make notes for each of the following questions, and then compare your response with the sample answer.
- If AST is performed in your workplace, are CLSI or EUCAST guidelines and standards used? What factors influenced this choice? Which version are you using, and are the guidelines up to date? How do you incorporate changes in methodology? If your workplace does not yet perform AST or is operating to an older set of guidelines, what factors would you consider if you had to decide which system to use? How might you set about introducing it?
- What might be the consequences of two laboratories using different guidelines and standards to each other?
Discussion
The choice of which system to use will be determined primarily by cost, availability of chemicals and materials, and geographical region.
Changing to a new set of standards is complex and time-consuming. It needs proper funding and full support from laboratory management. A biomedical scientist needs to be given time out from their day-to-day work to organise the change, and there also needs to be a system in place to check for (and incorporate) updates. Laboratory staff training is essential, as is good communication with all the stakeholders, such as the doctors who send in specimens and treat patients based on the results.
Because the CLSI and EUCAST reference standards for antimicrobial susceptibility are based on separate methodologies, they are not interchangeable. This means that laboratories using different systems may interpret the antimicrobial susceptibility of the same clinical isolate differently.
The potential for discrepancy means that, where possible, laboratories should only use one type of reference standard to interpret and report results that will guide antibiotic treatment decisions. There are also wider implications for both inter-country and global AMR surveillance if different systems are used. Additionally, the guidelines are updated on a regular basis (usually annually), so it is important to check which version has been used when comparing different sets of data.
Important
This module is based on EUCAST guidelines, as these are free and widely accessible, with occasional use of CLSI VET guidelines.
We do not recommend use of EUCAST human reference standards for veterinary clinical practice (i.e. in deciding which treatment to give an animal). Wherever possible, CLSI VET guidelines should be used to inform veterinary clinical decisions.
Remember that guidelines are updated regularly and you should check the CLSI [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] and EUCAST websites for the most up-to-date versions.
2 International guidelines for AST
