Severe Combined Immunodeficiency (SCID) is one of the most severe forms of Primary Immunodeficiencies (PID) and leads to a potentially fatal course of disease without early and definitive treatment. Adequate management, from the first days of life, can improve the survival and outcome of patients with SCID. This can be achieved through newborn screening (NBS) based on the measurement of T-cell receptor excision circles (TREC). Already present in many countries, this NBS test was introduced in Switzerland in January 2019 on a pilot phase. In addition to the assessment of TRECs, the measurement of kappa recombinant excision circles (KREC) has also been introduced at the same time and allows the identification of severe forms of PID characterized by profound B cell lymphopenia.