Tandem mass spectrometry (MS/MS) was introduced to expand newborn screening in Nuevo Leon, Mexico. This has permitted an increase in the diagnosis of many metabolic
disorders including isolated 3 methylcrotonyl CoA carboxylase deficiency (3 MCC).
Detection of an elevation of C5OH by MS/MS is associated with leucine catabolism disorders; false positive results are related with maternal transfer or immaturity of the
enzymatic systems. The confirmatory diagnosis is based on organic acids test in urine and decreased enzyme activity in fibroblasts. We report three cases of abnormal C5OH
in among 42 264 newborns (1:14000).1 The diagnosis was confirmed in only one child (~1:40 000). The incidence of 3 MCC deficiency in our state is similar to that of other
populations. This is the first report about the incidence of this disorder in newborns in a Mexican population
