Tag Archives: antimicrobial susceptibility testing

“Keeping it simple or keeping it accurate…”

Antimicrobial Resistance, The Science of Microbiology, , ,

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…

Time for some major disruption!

Michael

“Think twice”

Antimicrobial Resistance, ,

What have the following got in common?

  • E.coli resistant to nitrofurantoin
  • E.coli resistant to fosfomycin
  • Haemophilus influenzae resistant to ciprofloxacin
  • Group B streptococcus resistant to penicillin.
  • Coagulase negative staphylococci resistant to vancomycin
  • Candida albicans resistant to fluconazole

In my area of the world anyway (New Zealand), the percentage resistance rates of the above micro-organism/antimicrobial combinations is less than 1%. i.e. the prevalence is very low.

And because the prevalence is very low, unless your susceptibility testing methods are very specific, the positive predictive value of the result will also be very low. Thus , there will generally be a large number of false positives amongst such results.

Such a result should therefore automatically trigger a double check of everything, with a close look at the audit trail leading to the result. In some circumstances, repeating the test or sending the isolate to a reference laboratory may be the best option even if the result looks genuine.

We always need to be very careful when reporting low prevalence results, because even though we would like them to be, our tests are generally not perfect…

Michael

 

 

“Excuses”

The Art of Microbiology,

You might belong to one of the many labs/institutions that still routinely performs susceptibility testing on beta- haemolytic streptococci. 

Beta-haemolytic streptococci are invariably susceptible to penicillin, so unless there is a clinical history of allergy to penicillin, why bother?

Here are a few potential reasons:

  • Just in case the patient develops an allergy to penicillin, or the allergy was not mentioned on the request form.
  • That is the way we have always done it.
  • People might complain or be upset if we change.
  • It is easy to do it so we might as well.
  • It gives us epidemiological information (even though we are a diagnostic lab)
  • We might be the first lab to detect penicillin resistance.

All of the above can be potentially argued, but not very strongly, and in the meantime, someone foots the bill for virtually meaningless testing. I see the above more as excuses than reasons.

Sometimes we think the sky might fall if we change a method that is engrained in our psyche, but in my experience the sky rarely falls. In contrast such changes usually makes things brighter….

Michael

…A few labs might do it because they get paid for it (fee for service funding). However in a financially constricted Health Sector this is difficult to justify morally and demonstrates one of the main weaknesses of such a funding model.