Diagnosis of PKU Routine neonatal screening Phenylalanine levels (See also the American College of Medical Genetics and Genomics Therapeutic Committee's 2013 diagnosis and management guidelines for phenylalanine hydroxylase deficiency.) In the US and many developed countries, all neonates are screened for phenylketonuria 24 to 48 hours after birth with one of several blood tests; abnormal results are confirmed by directly measuring phenylalanine levels.