Neonatal Screening for Phenylketonuria: II. Age Dependence of Initial Phenylalanine in Infants With PKU

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Abstract
The relationships between initial and follow-up blood phenylalanine concentration and between initial concentration and age were examined in infants with elevated screening tests. Data were provided by health departments and phenylketonuria (PKU) clinics. Among infants who attained levels of 20 mg/100 ml or more the following was observed: (1) one-fourth had only minimal elevations (to less than 10 mg/100 ml) on the initial screening test; (2) of those with minimal elevations 66% were screened on or before the third day of life; (3) most of those whose initial elevations were more than minimally elevated and most of those with minimal elevations who were screened early had follow-up phenylalanines of 30 mg/100 ml or more. In contrast, those with minimal initial elevations who were screened late were more likely to have follow-up phenylalanines between 20 and 30 mg/100 ml. They may represent a different form of PKU. The findings suggest that as long as most infants in the United States continue to be screened on or before 4 days of age, some infants with PKU will be missed.

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