Phenylketonuria - Newborn Bloodspot Screening


Phenylketonuria is an inherited autosomal recessive condition.  Approximately one in every 4,500 babies born in Ireland have PKU or a milder form called hyperphenylalaninaemia.  When diagnosed within the newborn period and started on treatment, these infants will grow up to be healthy and well.  However, without treatment this condition may cause intellectual or physical disability. 

The condition is caused by a lack of the enzyme phenylalanine hydroxylase, which converts the amino acid phenylalanine into tyrosine.  In its absence phenylalanine accumulates and high levels have a direct toxic effect on the brain. 

Early treatment is beneficial and aims at giving a low intake of phenylalanine but a normal intake of all the other amino acids.  This diet has to be continued for life.  Early detection leads to early treatment.

The screening test depends on detecting a high level of phenylalanine in the blood.  If the test is carried out before 72 hours after birth there is a possibility that the level of phenylalanine in blood may not be sufficiently elevated for the condition to be detected.

The treatment for PKU has been one of the major successes in medicine since it was first introduced in early 1950s.  There is substantial evidence to show that the earlier treatment is started and the better the biochemical control throughout life, the better the outcome will be for the affected individual.

Action following detection of raised result

 When the bloodspot phenylalanine is raised, PKU/hyperphenylalaninaemia is suspected; the following clinical procedure is implemented:

1.      The test results are discussed with the On-call Consultant Metabolic Paediatrician and a decision made as to when the child should be admitted, either as an inpatient or, if the blood values are only moderately elevated, to the Outpatient Clinic.

  • The duty dietician is informed of the pending admission.

2.      The Clinical Liaison Officer (NNBSL) will:

  • book a bed through the Admission’s office in the Children’s University Hospital
  • contact the Maternity Unit in which the baby was born.  Clear verbal instructions are given to include the baby’s demographic details, the condition suspected and the arrangements for admission including the ward.  The Maternity Unit Contact is asked to give the mobile telephone number of the Director of the NNBSL to the parents; the parents are invited to contact the Director to obtain more information should they so wish.

3.      If the child is to be admitted the parents should present themselves with their baby to the Admission’s Office at the Children’s University Hospital if before 17.00 or otherwise go straight to the ward.

  • On admission the child will be examined by a doctor, the diet reviewed and a further blood test performed.  This is to confirm the diagnosis (plasma phenylalanine and tyrosine levels) and to determine the actual cause of the hyperphenylalaninaemia (blood pterins).
  • Depending on the initial blood phenylalanine level and the response to treatment the child will probably remain in hospital for about four days.  During this time the parents will meet all members of the clinical management team, become familiar with the condition, and learn how to take further samples.  This should be the only time the child should ever be admitted to hospital if the blood phenylalanine levels remain well controlled.
  • Not all infants with hyperphenylalaninaemia will be started on a low phenylalanine diet

4.      If the child is to attend initially as an outpatient, an appointment will be sent directly to the parents

Once the diagnosis has been confirmed the screening process is complete