“A little dose of antimicrobial resistance.”

Size matters when it comes to antimicrobial resistance…


I am talking here about the size of the antimicrobial dose that is given to the patient.

If a small dose is given to the patient then the drug levels at the site of infection may be at or just above the MIC of the drug to that particular micro-organism. This may allow mutants with slightly higher MIC breakpoints to be selected out in the presence of the antibiotic thus increasing anti-microbial resistance.

If a large dose is given to the patient, then the chances are much lower of any mutant strains having MICs higher than the antimicrobial level at the site of infection and thus surviving.

Dosing recommendations for adults are still very much in a one size fits all type category with a relatively small range recommended by the manufacturers and prescribed by the clinicians.

For the reasons above I always advise antimicrobial doses that are at the upper end of the recommended range. Unfortunately this does not always happen as doctors have a natural inclination to start off with a low dose and then increase if the anti-microbial is not working.


Antimicrobial dosing also has implications for the laboratory and the murky world of in-vitro versus in-vivo susceptibility. Our breakpoints essentially assume that every patient gets the same dose of antibiotic. Is there a better way?


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