Phenylketonuria (or PKU) is a rare inherited disorder where the baby has an absence, or low level, of a liver enzyme called phenylalanine hydroxylase. This enzyme is needed to break down part of the protein in the diet called phenylalanine, an amino acid (one of many amino acids that make up protein).
Our body cannot use phenylalanine until it is broken down by the phenylalanine hydroxylase enzyme to a substance called tyrosine. A lack of the phenylalanine hydroxylase enzyme causes a build up of the phenylalanine to toxic concentrations in the body that can cause permanent brain damage if not treated early.
There are no obvious physical signs of PKU. Brain impairment can be prevented or reduced if the baby is placed on a special phenylalanine-free formula within the first 20 days of life. Babies with PKU are unable to breastfeed.
PKU affects about 1 in 10,000 babies (or 0.0001%). The disorder can be carried by healthy parents and passed onto their baby (in 25% of cases), if both parents carry the gene for PKU. When both parents are healthy but carry the PKU gene they have a 25% chance of having a baby who has PKU, a 50% chance of having a baby who is a healthy carrier of the PKU gene (like themselves) and a 25% chance of having a baby who is neither a carrier nor a sufferer of PKU.
It is possible for parents with a family history of PKU to be tested to see if they carry the PKU gene. This can give them an opportunity to have genetic counselling prior to conceiving a baby or to consider genetic tests for their baby early in the pregnancy to see if they have PKU.
If a baby is born with PKU, the phenylalanine will start to accumulate within hours of the birth and reach relatively high levels within a week or so. The newborn screening test aims to detect PKU early, so that the baby can be placed on a special phenylalanine-free formula. This test has been available since about 1964.