Abstract: Described herein is a method for identifying a subject suffering from a severe form of a metabolic disease, including a) obtaining (i) at least partial nucleic acid sequences of at least two alleles of a gene contributing to the metabolic disease or at least partial amino acid sequences of the polypeptides expressed therefrom; and (ii) a residual activity of the polypeptides expressed from the at least two alleles, and b) based on the result of step a), identifying a subject suffering from a severe form of a metabolic disease. Also described herein are methods, data collections, devices, kit, and computer program products related thereto.