Let’s say you are a clinician and you are looking at two different microbiology results on two different patients (A&B). Both have an E. coli UTI. Both results state that the E. coli is susceptible to trimethoprim. However what you don’t know is that the E. coli isolate on Patient A had a trimethoprim disc diffusion zone of 18mm (right on the EUCAST breakpoint), whilst for patient B the corresponding zone was a much more comfortable 26 mm.
And who knows, if you repeated the same testing on patient A a dozen times, the chances are you would have a few “non-susceptible”results, due to the natural margin of error of the test.
If I was a clinician, and had this extra (zone diameter) information, I would be a lot happier prescribing trimethoprim to patient B, even though they both have in-vitro “susceptibility” reported on the result. (The same principle of course applies if we were talking about Minimum Inhibitory Concentration (MIC) values instead of zone diameters.)
But do clinicians really want this extra information?
They are usually very busy, …and not particularly interested in microbiology.
In my experience all clinicians generally want to know is if an isolate is susceptible or resistant. They are not particularly interested in the details, with the exception of blood culture and sterile site isolates, when there is at least a modicum of interest in the degree of susceptibility or resistance.
So which is better.. a susceptibility result full of information, but potentially difficult to understand and interpret, or a result reduced to its simplest form.
There is no right answer of course…
I am not even convinced antimicrobial susceptibility breakpoints have a long term future.
More and more, year by year, the anti-microbial susceptibility committees (EUCAST, CLSI) are trying to take into account antibiotic dose, renal function, degree of infection, etc. when setting antimicrobial breakpoints.
But they are really only scratching the surface…