Pitfalls in TDM of antibiotic drugs: analytical and modelling issues.

The quality assurance program of the Dutch KKGT [Association for Quality Assessment in therapeutic drug monitoring (TDM) and Clinical Toxicology] has been running for more than 25 years. One of these programs concerns TDM of the antibiotic drugs gentamicin, tobramycin, amikacin, and vancomycin. We present two issues encountered in a recent survey. In a case of gentamicin monitoring and dose-adjustment, a systematic analytical error in some centers led to a dosing recommendation that differed from that of the organizers. Correction of the analytical results on the basis of a standard control sample resulted in concentration differences of more than 20% and different dosing recommendations in these centers. In a case of vancomycin TDM, the choice of the population model proved to be critical for dose adjustment. We illustrate this example by presenting the plasma profiles derived from the different population models used by the participants.