15 Jul 2022

Selective susceptibility testing and reporting

EUCAST still receives comments from colleagues in clinical microbiology that when organisms are reported “I” (Susceptible, increased exposure”), clinical colleagues will favor agents for which the organism has been reported “S” (Susceptible, standard dose), thinking that an S is better than an I. One example most often brought forward is Pseudomonas aeruginosa where piperacillin-tazobactam, ceftazidime, ciprofloxacin, levofloxacin, imipenem (to mention some of the most common) will be reported “Susceptible, increased exposure", when the organism has no resistance mechanisms. There is a risk clinical colleagues favor meropenem, meropenem-vaborbactam, imipenem-relebactam, ceftolozane-tazobactam, ceftazidime-avibactam or cefiderocol because these are reported “Susceptible, standard dose” and that these agents are therefore unnecessarily over-used.

The remedies are:

  1. To explain to clinical colleagues that an “I” is as good as an “S” provided care is taken to choose the correct dose, a favorable dosing interval and an optimal mode of administration.
  2. To exercise diagnostic stewardship* and, as a part of this, reporting stewardship. Testing is one thing and reporting another. Not all tested agents need to be reported up front. When the report lists several standard agent alternatives such as piperacillin-tazobactam, ceftazidime and ciprofloxacin, do not report meropenem without or with vaborbactam, imipenem-relebactam, ceftazidime-avibactam, ceftolozane-tazobactam, cefiderocol and the likes – keep them in reserve and make them available on request only.

On the other hand, if colleagues have not fully understood the difficulties posed by serious Pseudomonas infections, and underdose standard agents such as piperacillin-tazobactam, ceftazidime or ciprofloxacin then it is better to use meropenem, ceftazidime-avibactam, ceftolozane-tazobactam etc where the standard dose takes into account the treatment of serious Pseudomonas infections.

*Selective reporting of antibiotic susceptibility testing results: less is more. G. Kahlmeter, Nathalie Thilly, Céline Pulcini. CMI, 2022. DOI: