[Pharmacokinetic and pharmacodynamic concepts for an interpretative reading of the antibiogram]

Both microbiological and epidemiological reading of the antibiogram can be performed without additional pharmacokinetic or pharmacodynamic concepts. However, when the aim is a clinical reading of the antibiogram, knowledge of the pharmacokinetics of antimicrobial agents and of all phenomena occurring between antimicrobial agents and microorganisms is imperative. Pharmacokinetics includes the study of absorption, distribution, metabolism and elimination of drugs. These data provide information on the active concentrations of antimicrobial agents in blood and other fluids as well as in the tissues where infection may develop. Knowledge of metabolism and of elimination pathways complete the main pharmacokinetic parameters of antimicrobial agents. The bactericidal effect of antimicrobial agents can be either concentration- or time-dependent. In the former, high concentrations of antimicrobial agents, much higher than their corresponding MICs, are needed, while in the latter, maintaining the concentration of antimicrobial agents at levels slightly higher than their MICs over time is more important. With these concepts, a microbiological-pharmacological reading of the antibiogram, taking into account dosage, administration pathway and location of the infectious process among other factors, can be achieved. Most working groups, either national or from abroad, have considered both pharmacokinetics and pharmacodynamics in interpretative reading of the antibiogram. In all cases, breakpoints for susceptibility and resistance should be corrected on the basis of data from well designed and performed clinical trials. Clinically oriented breakpoints do not necessarily have to be the same as those based on microbiological or epidemiological concepts, but clinical microbiologists should be able to give an appropriate response to the question and to the application based on that response.