What is a PKU Test?

Phenylketonuria (PKU) is an autosomal recessive metabolic disorder that increases the amino acid phenylalanine (Phe). This amino acid is usually found in diet and those with this inheritable condition have difficulty with breaking down Phe.

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Subsequently, this results in the build-up of Phe within the body and brain, ultimately leading to brain damage. The dysfunctionality in Phe metabolism is a result of a deficiency in the enzyme, phenylalanine hydroxylase (PAH), which prevents the effective breakdown of Phe, leading to severe complications.

PKU is usually diagnosed after a PKU test which is provided to babies after the first couple of days of birth due to the severity of this genetic condition. In the UK, approximately 1 in 10,000 babies inherit this disorder, while in the USA, the incidence rate is between 1 in 12,000 and 1 in 20,000 for Caucasians and Asians. The National PKU Alliance (NPKUA) has estimated that there are approximately 16,500 people who currently have this genetic condition within the USA.

To read more about newborn screening, click here.

Forms of PKU and hyperphenylalaninaemia are due to mutations within the PAH gene on chromosome 12q23.2. A PKU mutation, which is autosomal recessive, requires two copies of the mutated gene from both parents, to be found.

A diagnosis of PKU would require carrier parents who have one copy of the mutated gene themselves, producing a child with a 1 in 4 chance of inheriting the disorder. However, the child also has a 1 in 2 chance of being a carrier and a 1 in 4 chance of receiving a normal non-mutated set of genes from their carrier parents.

The importance of diagnosing this genetic disorder early through a PKU test ensures that it is managed appropriately, which can lead to healthy lifespans.

Significance of phenylalanine

Phenylalanine can be found in the diet through the ingestion of proteins as well as in artificial sweeteners; this amino acid is involved in the biosynthesis of other amino acids, whilst also providing structural and functional support to proteins and enzymes.

Phe is usually converted into a subsequent amino acid, such as, tyrosine, which is important for synthesizing dopamine and norepinephrine neurotransmitters. Phenylalanine can be categorized into two forms, L-phenylalanine, which is used for protein synthesis, and D-phenylalanine, which functions as a pain-killer. This is significant for optimum functioning of the brain, which can affect daily life and motor function and so a reduction of a neurotransmitter such as dopamine can lead to motor-neuron disorders.

Those diagnosed with this inherited metabolic condition must follow a restricted diet that limits the intake of phenylalanine, which would translate to a reduced protein diet. The combined daily requirement of both phenylalanine and tyrosine for adults is approximately 39 mg/kg, which should not be exceeded by adults with PKU.

If PKU is left untreated, resulting in an accumulation of Phe in the body, the amino acid is converted into phenylpyruvate, which ultimately leads to the development of seizures, brain damage, and mental retardation.

PKU test

A PKU test is provided to babies within hospitals during the first couple of days of birth to test for this condition. The newborn bloodspot screening test, which tests for PKU as well as other conditions involves pricking a baby’s heel to collect a few drops of blood for testing.

A confirmation and diagnosis of PKU in a newborn enable patients to be provided with immediate treatment to reduce the risk of developing severe complications. The treatment consists of a special low protein diet as well as amino acid supplements.

Additionally, regular blood tests are required to provide a method of monitoring phenylalanine levels in the body. Subsequently, this ensures the individual’s dietary plan or amino acid intake can be altered depending on their needs throughout their lives.

Aspartame and phenylalanine

Those who have been diagnosed with having a PKU genetic disorder must also avoid food products that contain aspartame due to its conversion into phenylalanine in the body. This artificial non-saccharide sweetener is used as a sugar substitute within tea or coffee, as well as chewing gum and the diet version of soda drinks.

This could potentially result in having a high level of Phe in the body, and without the effective breakdown of this amino acid by the enzyme phenylalanine hydroxylase, the adverse side effects of PKU can begin to develop.

PKU is an inherited recessive genetic condition that is usually diagnosed early through a PKU test that is provided by hospitals during the first week of birth through a blood collection via a heel prick.

Understanding the significance of this genetic disorder enables those who care for a child with PKU or an adult with PKU to manage their diagnosis appropriately, ensuring they do not falter with their restricted diet or routine blood tests.

Under a monitored and careful plan of managing high phenylalanine levels, those with this genetic disorder can lead long and fulfilling lives, without suffering any long-term complications.


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  • Mayo Clinic. 2021. Phenylketonuria (PKU) – Symptoms and causes. [online] Available at: https://www.mayoclinic.org/diseases-conditions/phenylketonuria/symptoms-causes/syc-20376302> [Accessed 17 September 2021].
  • NHS.uk. 2021. Phenylketonuria. [online] Available at: https://www.nhs.uk/conditions/phenylketonuria/ [Accessed 17 September 2021].
  • Npkua.org. 2021. NPKUA > What is PKU > About PKU. [online] Available at: https://www.npkua.org/What-is-PKU/About-PKU#:~:text=In%20the%20US%20the%20incidence%20rate%20appears%20to,frequency%20of%20occurrence%20between%20males%20and%20females%20%5B6%5D. [Accessed 17 September 2021].
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Further Reading

  • All Newborn Screening Content
  • What is Newborn Screening?

Last Updated: Oct 21, 2021

Written by

Marzia Khan

Marzia Khan is a lover of scientific research and innovation. She immerses herself in literature and novel therapeutics which she does through her position on the Royal Free Ethical Review Board. Marzia has a MSc in Nanotechnology and Regenerative Medicine as well as a BSc in Biomedical Sciences. She is currently working in the NHS and is engaging in a scientific innovation program.

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