Blood tests for Phenylketonuria (PKU)

Phenylketonuria (PKU): a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine (in high protein foods). 

PKU is inhereted and both parents must pass on the defective gene in order for a baby to have a condition.

Without the enzyme, levels of phenylalanine and two closely-related substances build up in the body. These substances are harmful to the CNS and can cause brain damage.

• Carried out when baby is around 5 days old.
• Midwife or paediatrician will clean the baby's heel with alcohol and collect a blood sample by pricking the heel with a special device and squeezing out a few drops of blood onto a card (Guthrie card).
• This may be uncomfortable and the baby may cry.
• The card is sent to a lab where levels of phenylalanine are measured.
• If levels are high, a second sample is taken to confirm the diagnosis.

Sample is tested for high levels of phenylalanine. If high levels are present it suggests that the infant has been born with PKU. 

If the baby has PKU, they may need testing as often as often as once a week for the first year of their life to check their phenylalanine levels. After that, they may have testing once or twice a month throughout their childhood.

PKU is rare - it affects 1 in 10,000 babies born in the UK.

Positive result: Person has the condition and will have to have further blood & urine testing to confirm the diagnosis. 

Negative result: There are low levels of phenylalanine and baby does not have PKU.