All newborn babies have a blood test to check for PKU 

What is phenylalanine?

Phenylalanine is an amino acid, which is a 'building block' of protein. It is found in high-protein foods such as:

  • meat
  • fish
  • eggs
  • cheese
  • milk

Phenylketonuria is usually treated with a diet that avoids high protein foods.

Phenylketonuria (PKU) is a rare genetic condition that is present from birth. The body is unable to break down a substance called phenylalanine, which builds up in the blood and brain.

High levels of phenylalanine can damage the brain. PKU is treated with a special low-protein diet, which reduces the levels of phenylalanine in the body and prevents brain damage.

PKU doesn't usually cause any symptoms if treatment is started early – but if it isn't treated, damage to the brain and nervous system can lead to:

  • learning disabilities
  • behavioural difficulties
  • epilepsy

High levels of phenylalanine can also cause eczema.  

Read more about the symptoms of phenylketonuria.

What happens?

The body takes in phenylalanine in the form of protein in food. Phenylalanine is normally broken down in the body by an enzyme called phenylalanine hydroxylase.

In people with PKU, this enzyme does not work properly because of a genetic mutation (alteration in their DNA). As a result, phenylalanine levels in their blood, and other tissues, rise.

Read more about the causes of phenylketonuria.

How common is it?

PKU is rare – it is estimated to affect 1 in every 10,000 babies born in the UK.

Both sexes are affected equally by PKU. The condition is slightly more common in white people, particularly those of Celtic descent (Scottish, Irish and Welsh). It is very rare in black and Japanese people.

Screening for PKU

All babies born in the UK and other developed countries are routinely screened for high phenylalanine levels. This is done using the heel prick test and is carried out during the first week of a baby’s life.

During the test, a blood sample is taken from the baby’s heel to test for a number of rare but serious conditions, including PKU.

If phenylketonuria is picked up, further tests will be arranged to confirm the diagnosis.

Read more about diagnosing phenylketonuria.

Treating PKU

Without treatment early in life, most people with PKU will develop severe learning difficulties and will require constant care.

The main treatment for PKU is a low-protein diet that avoids foods such as meat, eggs and dairy products, and controls the intake of many other foods (such as potatoes and cereals). In addition, patients must take an amino acid supplement to ensure that they are getting all of the nutrients required for normal growth and good health.

There are also a number of specially designed low-protein versions of popular products, such as flour, rice and pasta specifically designed for people with PKU and related conditions, many of which are available on prescription.

If a high phenylalanine level is confirmed, a baby will immediately be started on a low-protein diet and amino acid supplements. Phenylalanine levels in the blood are regularly monitored by collecting blood from a finger prick onto a special card and sending it to a laboratory.

As long as a person with PKU sticks to a low-protein diet throughout childhood and their phenylalanine levels stay within certain limits, they will remain well and their natural intelligence will be unaffected.

Adults with PKU

It is generally accepted that adults with PKU probably function best while on the low-protein diet.

However, unlike children, high phenylalanine levels are not known to cause permanent damage to the brain in adults with PKU.

For this reason, clinicians differ in opinion over whether it is strictly necessary for adults with phenylketonuria to continue with the low-protein diet for the rest of their life.

Some adults with PKU find it difficult to follow the low-protein diet and attempt to come off it. They may find that as a result, they don't function as well - for example, they may lose concentration and have a slower reaction time. Others don't seem to have any problems.

Any adverse effects caused by coming off the low-protein diet can usually be reversed by going back on the diet.

Any patients who attempt to come off the diet should be supported by their clinician and should be followed up regularly, to monitor their condition for any complications that might arise.

Read more about the treatment of phenylketonuria.

Page last reviewed: 25/09/2012

Next review due: 25/09/2014


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The 6 comments posted are personal views. Any information they give has not been checked and may not be accurate.

PKU Parent said on 27 April 2014

I cannot believe some of the things written on here!. After some monitoring of individuals as children is done sufferers are left to drift out of treatment in their adult lives. The NHS has no idea about the effects of the condition in adulthood as is doesn't keep track of where the patients are let alone the affects that the condition is having upon them. If adults don't attend sessions it is assumed that it is their informed choice and no follow up is attempted despite the fact that one of the many symptoms of high phenylalanine levels is psychological disturbance, depression and confusion. No one can say long term damage doesn't occur in adulthood because you can't find that said definitively across all of the publications in journals. This means when you speak to consultants all of their information is purely anecdotal and doesn't include the experiences of the patients they don't see ie those off diet. Please can we be more honest on sites like these read by the parents of pku new borns. Your child will be safe and grow up to be wonderful happy and normal... but treatment for this condition in the UK needs to improve both in terms of the use of new drug treatments and in follow up care and support into adulthood

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Katie777 said on 18 February 2014

When it says that all babies are tested for this - can you tell me, approximately, what year testing for this started? Is it possible older people may have not had this test?

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lizzie37 said on 09 January 2014

Adults should not go off diet, this information is incorrect. All patients in all countries are being told to stay on the diet the effect on the brain can be irreversible . My daughter has had at least two GPs that don't have a clue about PKU and think that she will grow out of it. It is a genetic disorder for goodness sake not bad habit. PKU sufferes have the condition for life and the condition must be treated for life in order to avoid the following which is by no means exhaustive list of side effects of eating a normal diet:

increased occurrences of eczema, a skin rash which has been linked to PKU

a tremor in the hands and eyelids

memory problems

lack of attention span and concentration

raised levels of anxiety and depression

hyperactivity and irritability

social disorders, i.e. agoraphobia

decreased motor functions, i.e. hand-eye coordination

on average, a loss of one IQ point per year off diet

The treatment of PKU includes a low protein diet, amino supplements, and prescribable foods to replace those that can't be consumed. If the sufferers didn't get the prescribable foods they would be malnourished. YOu must mention this and also that correct blood levels of phe are maintained by regular blood test done at home.

Please would someone speak to a Metabolic Consultant and get this page looked at it is filled with inaccuracies, which is affecting how GP's approach the condition and the patient.

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KitRen said on 20 April 2012

Do the nhs offer to urine test screening instead of the heel prick test?

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RosieTalk said on 01 March 2011


This page has now been corrected. Thank you for your input.

Rosie, moderation NHS Choices

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bob5211 said on 20 February 2011

The enzyme is called phenylalaninehydRoxylase.

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Newborn screening tests

When a baby is born, a range of routine tests are on offer to pick up certain health problems early