The objective in this new project is to optimize the therapeutic regimen of pediatric patients to maximize efficacy and minimize side effects. This will be achieved by developing a mass spectrometry-based breath test.Ultimately, this will lead to new opportunities to guide the dosage of drugs with high accuracy, in real-time and in a patient-friendly fashion.We will use a cutting-edge analytical platform (i.e. secondary electrospray ionization-mass spectrometry) available at the University Children’s Hospital Basel to detect drugs in breath with unparalleled speed, selectivity and sensitivity.Initially, we will measure simultaneously blood and breath concentrations of drugs routinely monitored in our hospital(e.g. anti-convulsants). We will then use this information as an input to develop pharmacokinetic computational models to predict blood concentrations based on the breath test read-out.During the final phase, we will validate these models in an independent group of patients to proof the clinical transferability of breath-based tests to guide drug dosage on an individual basis. This project will have a tremendous impact on current pediatric TDM clinical practice by: i)enabling a more personalized treatment, hence reducing ineffective doses and adverse effects.