Blood Screening for Phenylketonuria

To the Editor:— Since the discovery by Fölling, in 1935, of phenylketonuria (PKU), the simple ferric chloride test of urine has been the universal method of detecting this disease in spite of serious limitations. These include (1) failure of young infants to produce a positive test until several weeks after birth, even though there is evidence that blood phenylalanine concentration increases rapidly in the first few days of life in this condition (Armstronget al., Proceedings of the Third International Neurochemical Symposium,1958, to be published) and (2) failure of older PKU cases to produce a positive ferric chloride test if blood phenylalanine levels are below approximately 20 mg. per cent. Obviously a simple method of detecting elevated phenylalanine blood levels should overcome these limitations. Recently, with the cooperation of Dr. William Welch, Director of the Laboratory at Newark State School, Newark, N.Y., we have had the opportunity to develop