A mandatory external quality assessment scheme for Therapeutic Drug Monitoring is operational in Belgium. This quality scheme aims to provide an assessment of the state of the art of Therapeutic Drug Monitoring as practiced in the Belgian laboratories. The external quality assessment scheme' main objective is to evaluate laboratories on their ability to approach their peer group's median value. In addition to the analytical result, an interpretation for some parameters such as aminoglycosides is frequently asked by using case reports. Even when analytical performance is good, many laboratories encounter problems interpreting the results especially when the intervals of blood collections are uncommon. Results illustrate the added value of the use of dedicated software to support pharmacokinetic interpretation