Rapid diagnosis of phenylketonuria by quantitative analysis for phenylalanine and tyrosine in neonatal blood spots by tandem mass spectrometry.

A new method for quantifying specific amino acids in small volumes of plasma and whole blood has been developed. Based on isotope-dilution tandem mass spectrometry, the method takes only a few minutes to perform and requires minimal sample preparation. The accurate assay of both phenylalanine and tyrosine in dried blood spots used for neonatal screening for phenylketonuria in North Carolina successfully differentiated infants who had been classified as normal, affected, and falsely positive by current fluorometric methods. Because the mass-spectrometric method also recognizes other aminoacidemias simultaneously and is capable of automation, it represents a useful development toward a broad-spectrum neonatal screening method.