Diagnosis of Phenylketonuria (PKU):

Phenylketonuria is diagnosed when high levels of phenylalanine are found in the blood. Testing for PKU is generally done with a screening blood test within 48 hours of birth. Babies who test positive on the initial screening test will be evaluated again with a more specific test for PKU.

Phenylalanine levels may be normal at birth but go up once the baby is feeding. It is important to evaluate babies after they have received dietary protein for 24 to 48 hours. The practice of discharging babies from the hospital within 24 hours of delivery has resulted in failure to detect some infants with PKU.