3.2 Frequency of AMR infections

While proportions provide a relative measure of prevalence, frequencies describe the absolute rate of infections over a specific time period. Specifying the denominator of the frequency of AMR infections is important for interpretation. Generally, there are two types of denominators:

• population size within the catchment area
• tested population.

Frequency of AMR infections per 100,000 population per year would be defined as the ratio of the number of patients in the target population with infections caused by a resistant pathogen to the population size within the same target population over a specific period of time. This can be written as follows:

Similarly, frequency of AMR infections per 100,000 tested patients per year would be defined as the ratio of the number of patients in the target population with infections caused by a resistant pathogen to the total number of patients who had at least one clinical sample sent for microbiology testing within the same target population over a specific period of time. This could be written as follows:

For example, in some of the reports or publications you may encounter similar statements such as ‘the frequency of 3GCR Escherichia coli per 100,000 population in 2025 in country X was 5.8’, or ‘the frequency of 3GCR Escherichia coli per 100,000 tested patients in 2025 in Country X was 660’. Both morbidity measurements have the same numerator but different denominators.

Other commonly used terminology includes:

• frequency of AMR infections per 1000 patient days per year
• frequency of AMR infections per 1000 discharge events per year.

Both of these share the same numerator as described above but different denominators.

The term ‘incidence rate of AMR’ is used in the literature and generally refers to the number of new AMR infections occurring in a specific time period. It is conceptually similar to the ‘frequency of AMR’ but specifically focuses on newly identified cases.