In collaboration with the University of Bern, we have developed a novel proprietary mast cell line, which we continously grow and maintain in our laboratories. Using standardized protocols, we can now reliably produce any desired number of mast cells for our Functional Allergy Screening Test by Passive Sensitization (FAST-PASE^®). In FAST-PASE^®, mast cells are incubated with patient-derived blood serum or plasma to let the patient IgE (immunoglobulin E) adhere to its receptors on the cell surface. This step is referred to as passive sensitization. Subsequently, the sensitized mast cells are challenged with the allergens of choice.In case there is allergen specific IgE in the patient derived serum or plasma, stimulation with the culprit allergen(s) will result in activation and degranulation of the mast cells. This dose-dependent activation of the cells is measured in a quantitative manner by flow cytometry. During the entire procedure patient serum remains present, thereby taking into account all other factors present in the serum that may affect a patient’s response to the allergen(s). Our engineered mast cells are highly sensitive and show maximal degranulation reactivity of over 95%, while background signals remain below 1%, resulting in a very wide diagnostic window. FAST-PASE^® is a functional in vitro assay that closely reflects the in vivo situation, providing the physician with quantitative data about sensitivity and reactivity of the patient to easily identify culprit allergens and exclude non-reactive allergens.