Phenylketonuria masked by low protein feeds.

Two patients with phenylketonuria (PKU) requiring treatment were fed on low protein milks. Both had blood phenylalanine levels below 1200 micronmol/l (20mg/100 ml) until given a phenylalanine challenge. Phenylalanine content of mature breast milk may provide intakes similar to those used in treating PKU. Diagnosis of PKU is unlikely to be missed if screening is carried out on the sixth or seventh day of life because of higher phenylalanine in breast milk during the first week. Interpretation of screening tests requires knowledge of the infants' feeds and a blood phenylalanine above 360 micronmol/l (6 mg/100 ml) in the absence of tyrosinaemia requires careful investigation.