3.1 Proportion of AMR infections
Throughout this course, you will see the term ‘
In other words, it is the fraction of patients with a clinical sample culture positive for a resistant phenotype among the total number of patients with at least one positive clinical sample culture for the given organism over a specific period of time.
For example, the proportion of third-generation cephalosporin-resistant (3GCR) Escherichia coli among patients with at least one positive clinical sample culture for E. coli and tested against third-generation cephalosporin (3GC) in 2025 could be calculated using the following equation:
Suppose that among 100 patients with E. coli infections, all their E. coli isolates were tested for susceptibility to 3GCs. Of these, 15 patients had isolates that were resistant to 3GC. The proportion of 3GCR E. coli infections in this cohort can be calculated as:
15/100 = 0.15 or 15%
The term ‘prevalence of AMR’ is also sometimes used in literature to mean ‘proportion of AMR’. ‘Proportion of AMR’ is used in this course to remind you that this measurement of AMR burden is bounded between 0 and 1 (or between 0% and 100% in percentage terms). More importantly, ‘prevalence’ refers to the number of existing disease cases in a given population within a window of time. This is different from ‘incidence’, which refers to the number of new disease cases within a window of time.
One important caveat for proportion of AMR measurements is that a proportion of 30% could reflect three patients with 3GCR E. coli infections in a total number of ten patients with E. coli infections tested for resistant to 3GCs (with high uncertainty and likely a non-representative cohort), or 300 patients with 3GCR E. coli infections in a population of 1000 patients with E. coli infections (with lower uncertainty). Hence it is important that you explicitly report both the numerator (the number of patients with AMR infections) and the denominator (the total number of patients with bacterial infections) when describing the proportion of AMR, or that every time you encounter a reported proportion of AMR, you should ask for the numerator and denominator to support your interpretation.
Moreover, it is important to note that the unit of measurement needs to be consistent for the numerator and denominator. The commonly used units of measure include the number of samples and the number of patients.
A hospital has the following data for the year 2024:
- There were 550 patients who had an E. coli bloodstream infection.
- Of these 550 patients, 450 patients had antimicrobial susceptibility test (AST) results.
- Of these 450 patients who had AST results:
- 150 patients had E. coli isolate(s) identified from at least one clinical sample as resistant to 3GC
- 300 patients had clinical samples with E. coli isolates susceptible to 3GC.
Using the information above, to the nearest whole number what is the estimated proportion of 3GCR E. coli in bloodstream infections at the hospital?
- a.27%
- b.33%
- c.81%
- d.55%
Answer
The correct answer is option (b): 33%. 450 patient samples were tested and of these 150 were found to be resistant to 3GC. The proportion of AMR is calculated as:
3 Health outcomes of AMR infection
