We will perform a cluster-randomised controlled trial at 20 GP out-of-hours centres in the Netherlands. GP out-of-hours centres will be stratified by size, to ensure equal distribution of size between the intervention and control group. The required number of clusters and participants was based on the following assumptions: (1) ICC of 0.01, (2) alpha of 0.05, power of 0.80, (3) proportion of antibiotic prescriptions in control group of 25% and a proportion of 19% in the intervention group (6% minimal clinical relevant difference) and (4) 10% loss to follow-up and 10% efficiency loss based on unequal cluster sizes. Based on a previous cohort study, we estimated to include 1000 children per cluster (GP out-of-hours centre) within six months, resulting in a need for 20 clusters and an effective sample size of 737 patients in the intervention and control group (1474 in total).