This protocol outlines a two-part study that will establish a COS for each of two of the most common IMD in children, phenylketonuria (PKU) and medium-chain acyl-CoA dehydrogenase (MCAD) deficiency (our study is registered in the COMET database, http://comet-initiative.org/studies/details/995). This evidence concerns the gene ACADM and phenylketonuria.