What Does a Positive PKU Test Mean?

Medically Reviewed on 11/16/2021
Phenylketonuria PKU Test
Positive phenylketonuria (PKU) test looks for phenylalanine levels in the blood of more than 4 mg/dL, which is considered excessive and could indicate PKU.

Phenylketonuria (PKU) test or Guthrie assay is used to screen neonates (babies) for PKU, which, if not detected early and left untreated, can result in brain damage and severe intellectual disabilities.

Test results may differ depending on your child’s age, gender, medical history, test method, and other factors.

Findings of your child’s tests may not indicate that they have a problem. Hence, the child’s test results should be discussed with a healthcare provider.

  • Positive phenylketonuria (PKU) test: The test looks for phenylalanine levels in the blood, which should be less than 2 mg/dL. A blood phenylalanine level of more than 4 mg/dL is considered excessive and could indicate that the child has PKU. The test will be performed 24 hours after the baby is born and then again when your baby is 7 to 14 days old.
  • False-positive PKU test: False positives can occur if the baby is born prematurely because certain liver enzymes have not fully evolved, and this could result in a false-positive result.
  • False-negative PKU test: If the child is having feeding issues, such as vomiting, it could result in a false-negative result.

Medicines such as aspirin and antibiotics might skew the results of a PKU urine test.

What is PKU?

PKU is a hereditary or genetic disorder where people lack the enzyme required to metabolize the amino acid phenylalanine, which is found in many foods as part of proteins. Phenylalanine can build up to harmful quantities in the body without the presence of the enzyme to break it down.

PKU usually manifests itself in the first year of life, with infants appearing abnormally sleepy and listless. Infants may have difficulty eating and develop a red, itchy rash that resembles eczema. Furthermore, such babies usually have lighter skin and hair than their siblings and cousins who do not have the illness.

In addition, people with PKU excrete phenylacetic acid in their urine and sweat. They have a peculiar musty odor if the condition is not addressed. People with PKU must follow a diet that restricts the amount of phenylalanine they can consume, starting in childhood and continuing throughout their lives.

PKU was the first disorder to be tested in infants and is tested on all newborns in the United States, which indicates that practically all instances have now been identified and treatment has begun.

When should a PKU test be done?

The test should not be performed until 24 hours after the baby is born. If you're breastfeeding, tell your doctor about all medications, herbs, vitamins, and supplements you take. This covers over-the-counter medications and any illegal substances.

Why is the PKU test done?

This test may be necessary for your child because detecting and treating phenylketonuria (PKU) can help them avoid intellectual disability and other developmental difficulties.

  • Your child is more likely to have a healthy life if they eat a low-protein, low-phenylalanine diet in their first weeks of life and afterward.
  • Although most newborns with PKU are discovered soon after birth, any child with an intellectual handicap or who is developmentally delayed should be screened for PKU.
  • In the first year of life, some babies adopted from foreign countries may need to be tested for PKU and other genetic diseases.
  • A PKU test is done for all newborns in the United States.

How is the PKU test done?

A heel-prick test is commonly used to screen babies for phenylketonuria. A few drops of blood from the infant's heel are used for this.

A urine test can be used instead of a heel-prick test. The healthcare provider will take a sample of your baby’s urine.

What are the risks of the PKU test?

There could be some transient risks during the heel-prick test, such as bleeding, infection, and bruising. A mild discomfort, sting, or soreness may occur when the needle pricks your baby's heel, which doesn’t last long.


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Medically Reviewed on 11/16/2021
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