Tandem mass spectrometry as screening for inborn errors of metabolism The use of tandem mass spectrometry for the diagnosis of inborn errors of metabolism has the potential to expand the newborn screening panel to include a vast number of diseases. This technology allows the detection, in the same spot of dried blood on fi lter paper and during one single analytical run, of different metabolic diseases. Tandem mass spectrometry is rapidly replacing the classical screening techniques approach of one-metabolite detected per analysis per disease by its ability of simultaneous quantifi cation of several metabolites as markers of many diseases, such as acylcarnitines and amino acids. It is clear that a single metabolite can be a biomarker for several diseases, so the multiplex approach of using tandem mass spectrometry enhances, on average, the sensitivity and specifi city of the screening. However, there are differences for particular metabolites and the diseases they detect within the same method. Disorders such as the tyrosinemias and among the organic acidemias, the methylmalonic acidemias, have a substantially higher false-positive rate than other more common metabolic diseases such as medium-chain acyl-CoA dehydrogenase defi ciency and phenylketonuria. Before introducing this technology into routine newborn screening programs it is necessary to consider the frequency of each disease, as well as the response to early treatment or variables related to the collection of the sample.