Patients with the same disease can respond differently to the same medicine. To reduce the level of ‘trial and error’ in the treatment, prescription of drugs is increasingly accompanied by diagnostic testing to monitor the efficacy and safety of the treatment. BioCDx aims to develop a disruptive companion diagnostic device for early cancer diagnosis and treatment monitoring. One of the scientific breakthroughs of this project is the development of a detection platform for cancer stem cells, the most aggressive/tumorigenic cell type in tumors. BioCDx will rely on ultrasensitive, photonic elements based on an array of photonic biosensors. A nanoparticle-enhanced sandwich assay will be developed to detect circulating proteins, bringing the limit of detection to the femtomolar range. The detection platform will be integrated into a portable, desktop device and validated for three cancer types: breast cancer, prostate cancer and melanoma.