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Phenylketonuria (PKU)

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What is phenylketonuria?


VOL: 99, ISSUE: 30, PAGE NO: 26

What is phenylketonuria? 

- Phenylketonuria (PKU) is an inherited, autosomal recessive disorder, which affects the way proteins are metabolised.

- It is caused by a defective enzyme, which should convert the amino acid phenylalanine into tyrosine.

- This results in a build up of phenylalanine, a state called hyperphenylalaninaemia.

- Hyperphenylalaninaemia affects normal growth and development, especially of brain tissue.


- The aim of screening is to detect the presence of PKU sufficiently early to enable the initiation of treatment and the prevention of mental retardation and later morbidity.

- A blood screening test for PKU was developed in the USA by Guthrie and was introduced into the UK in the late 1960s.

- The test is now offered to all infants between six and 10 days old.

- Confirmed abnormal results should also be notified to the PKU Register. 

- Some families may require formal genetic counselling and should be referred to a clinical geneticist.


- One of the enzyme defects that cause PKU is present in approximately 1/10,000 births in northern Europeans.

- One person in every 50 of the general UK population carries a mutated copy of the affected gene.

- Incidence varies according to country and race and is 1/4,500 in Ireland, 1/16,000 in Switzerland, 1/50,000 in US blacks and 1/15,000 in US Caucasians (Hofman et al, 1991).

- Parents who have a child with PKU have a one in four risk of any subsequent children being affected. 


- The combination of screening and a low phenylalanine diet has greatly improved the prospects for those with PKU.

- When diagnosis is confirmed a special low phenylalanine feed is given until levels of phenylalanine fall to within the treatment range (this is higher than the normal reference range). This reduction in level usually takes a few days.

- Small amounts of breast or formula milk are then introduced to provide a limited amount of phenylalanine.

Dietary management

The low phenylalanine diet consists of three parts:

- A protein substitute, which includes all other amino acids except phenylalanine and an additional vitamins and minerals supplement. 

- A small measured quantity of phenylalanine, from foods such as potatoes, baked beans and breakfast cereals (Cockburn, 1993).

- Foods low enough in phenylalanine to be permitted in normal quantities (there are only a few, which include most fruit and vegetables, sugar, butter, boiled sweets and some squashes). In addition many specially produced low-protein products such as low protein flour, bread and pasta are available on prescription.

Nursing implications

- It is recommended that people with PKU follow the special diet for life.

- All immunisations can proceed normally.

- If a child’s phenylalanine control is kept within the acceptable range, growth and development will not be affected. Therefore, extra developmental assessments should not be required.

- After diagnosis the family may need extra support in coping with shock, anxiety or stress.

- It is important that dietary advice is consistent. The child’s dietitian should, therefore, be the only person giving advice.


National Society for Phenylketonuria 

Further reading 

Burton, H. et al (2003)Genetics education for nurses, midwives and health visitors. Professional Nurse; 18: 12, 676-680 (August edition).

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