Endocrinology and Metabolism: Long-Term Stability of Amino Acids and Acylcarnitines in Dried Blood Spots

newborn screening, are often stored for long periods of time. They may be suitable for the retrospective diag-nosis of inborn errors of metabolism, but no data are currently available on the long-term stability of amino acids and acylcarnitine species. Methods: We analyzed amino acids and acylcarnitines by tandem mass spectrometry in 660 anonymous, ran-domly selected filter cards from 1989 through 2004. We assessed long-term stability of metabolites by linear regression and estimated annual decrease of concentra-tion for each metabolite. Results: Concentrations of free carnitine increased by 7.6 % per year during the first 5 years of storage and decreased by 1.4 % per year thereafter. Alanine, arginine, leucine, methionine, and phenylalanine decreased b