What Is the PKU Test and How Is it Different for Preemies?

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The newborn screening test, often called the PKU test, is a blood test that looks for several different disorders in newborn babies. The PKU test is done by pricking a baby's heel and allowing several drops of blood to drip onto a special card. The test varies by state, with some states looking for more disorders than others.

The PKU test should be done after the infant is at least 24 to 48 hours old but within the first 7 days of life. It is usually done before a newborn baby leaves the hospital.

Parents of babies who were born at home or in another out-of-hospital setting should contact their midwife or pediatrician to find the closest lab that can do the newborn screening test.

Why Is the Newborn Screen Also Called the PKU Test?

In 1963, Dr. Robert Guthrie developed a simple and inexpensive test for phenylketonuria (PKU), in infants. Over time, the same test became used to look for many other disorders. Although the newborn screening test now looks for over 50 different disorders in some states, it is still often called the PKU test.

What Can the PKU Test Diagnose?

Although the PKU test varies by state, all states use the newborn screening to test for at least 21 different disorders, including:

  • PKU
  • Congenital hypothyroidism
  • Galactosemia

Most states use the PKU test to test for additional disorders with some testing for more than 50 different illnesses. Many of these disorders are metabolic, meaning that they affect the way the cells make energy.

Why Is the PKU Test Important?

The PKU test looks for disorders that can cause severe health problems if not treated early. Some of the disorders can be very serious or even life-threatening in the first week of life if not diagnosed and treated right away. Others don't show symptoms for months or even years, and the symptoms may look like other disorders.

How Is the PKU Test Different in the NICU?

Both the immaturity of preemies and the complications of NICU treatment can make the results of newborn screening tests more difficult to interpret. NICU patients may need to be screened more than once to make the results easier to understand.

Premature babies and term babies who require NICU care have several issues that may affect the newborn screening test:

  • Late feeding: Many NICU babies are not fed milk products right away, due to the risk of NEC, aspiration, or other issues. Unless advanced screening methods are used, results of the PKU test are most accurate after the baby has been receiving milk feedings for at least 24 to 48 hours but within 7 days of life.
  • Antibiotic therapy: Antibiotics can affect the accuracy of some of the tests performed on the PKU test.
  • Total parenteral nutrition (TPN): TPN can affect the accuracy of some of the newborn screening tests.
  • Blood transfusions: Because blood transfusions replace some of the baby's own blood with blood of a different composition, blood transfusions can affect make PKU tests less accurate.
  • Higher rates of hypothyroidism: Premature babies have higher rates of hypothyroidism than term babies. This condition is usually short-lived and gets better on its own quickly without any lasting effects, but can interfere with growth and development if not monitored closely.

If your baby was premature or spent time in the NICU, it is likely that he or she experienced some of the above interventions or had a period of hypothyroidism. NICU staff will usually do one PKU test before a planned intervention, and a repeat test will need to be drawn at a later time. A repeat test will also help determine if hypothyroidism will need to be treated.

Premature babies usually receive their repeat tests in the NICU. If you had a term baby who was in the NICU for a short period of time, ask your pediatrician if the PKU test should be repeated, and when.

What if the Newborn Screening Test/PKU Test is Positive?

If your baby's newborn screen was positive for PKU or another disorder, follow-up testing will be done to confirm the suspected diagnosis. Some newborn screening test results are false-positives, so your baby may not actually have the suspected disorder. If a disorder is identified, treatment will then be discussed and started.

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Article Sources
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  • Bryant, Kristin RN BSN MSN; Horns, Kimberly RNC, NNP, PHD; Longo, Nicola MD, PhD; Schiefelbein, Julieanne MappSc, RNC, MA, RM, PNP, NNP. "A Primer on Newborn Screening." Advances in Neonatal Care October 2004; 4, 306-317.
  • March of Dimes. "Newborn Screening Tests."
  • March of Dimes. "States Expand Newborn Screening for Life-Threatening Disorders."