RESULTS OF SCREENING FOR PHENYLKETONURIA USING A LOWER CUTOFF VALUE IN EARLY COLLECTED SPECIMENS

Introduction: Currently, 64% of the newborn blood specimens in California are collected on or before 1 day of age and approx. 16% are collected before 12 h of age. During the period 1990-91, the California Newborn Screening Program for phenylketonuria (PKU) instituted a lower phenylalanine cutoff for early-collected specimens because of the expressed concern in some reports that missed cases could result from lower elevations that infants with PKU might have during the first hours of life. Methods: During this period, 596 329 specimens were screened for PKU using the fluorometric method. The phenylalanine cutoff for all specimens whose collection age was at or under 12 h, or not recorded, was lowered to 4.0 mg/dl, from the standard cutoff of 4.3 mg/dl. Results: No cases of either classical or variant PKU were detected among a total of 1256 infants determined to be presumptive positive based on the 12 h or less, or unknown, collection age (n = 72 868) and a phenylalanine level of 4.0-4.2 mg/dl. One infant diagnosed with transient hyperphenylalaninemia and who has not required treatment was detected. This may be compared to the 24 confirmed cases of PKU and 19 variants detected among 1532 infants determined to be presumptive positive based on the 4.3 mg/dl cutoff, regardless of age. Discussion: Using a single 4.3 mg/dl cutoff, there have been no known cases of phenylketonuria missed by the California Newborn Screening Program since its inception in 1980, during which time over seven million infants have been screened for PKU. In view of the additional false positives generated, there does not appear to be justification for using a lower cutoff value in early discharged infants when a quantitative technique is used.