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Serum phenylalanine screening

Serum phenylalanine screening is a blood test to look for signs of the disease phenylketonuria (PKU). The test detects abnormally high levels of an amino acid called phenylalanine.

How the Test is Performed

The test is usually done as part of routine screening tests before a newborn leaves the hospital. If the child is not born in a hospital, the test should be done in the first 48 to 72 hours of life.

An area of the infant's skin, usually the heel, is cleaned with a germ killer and punctured with a sharp needle or a lancet. Three drops of blood are placed in 3 separate test circles on a piece of paper. Cotton or a bandage may be applied to the puncture site if it is still bleeding after the blood drops are taken.

The test paper is taken to the laboratory, where it is mixed with a type of bacteria that needs phenylalanine to grow. Another substance that blocks phenylalanine from reacting with anything else is added.

See also: Newborn screening tests

How to Prepare for the Test

For help preparing your baby for the test, see infant test or procedure preparation (birth to 1 year).

How the Test will Feel

When the needle is inserted to draw blood, some infants feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. Infants are given a small amount of sugar water, which has been shown to reduce the painful sensation associated with the skin puncture.

Why the Test is Performed

This test is done to screen infants for PKU, a fairly rare condition that occurs when the body lacks a substance needed to breakdown the amino acid phenylalanine.

If PKU is not detected early, increasing phenylalanine levels in the baby will cause intellectual disability. When discovered early, changes in the diet can help prevent the severe side effects of PKU.

Normal Results

A normal test result means that phenylalanine levels are normal and the child does not have PKU.

Normal value ranges may vary slightly among different laboratories. Talk to your health care provider about the meaning of your baby's test results.

What Abnormal Results Mean

If the screening test results are abnormal, PKU is a possibility. Further testing will be done if the phenylalanine levels in your baby's blood are too high.


The risks of having blood drawn are slight, but include:

  • Excessive bleeding
  • Fainting or feeling lightheaded
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)
  • Multiple punctures to locate veins

Alternative Names

Phenylalanine - blood test


Pasquali M, Longo N. Newborn screening and inborn errors of metabolism. In: Burtis CA, Ashwood ER, Bruns DE. Tietz Textbook of Clinical Chemistry and Molecular Diagnosis. 5th ed. St. Louis, MO: Elsevier Saunders; 2012:chap 58.

P. In: Chernecky CC, Berger BJ. Laboratory Tests and Diagnostic Procedures. 6th ed. St. Louis, MO: Elsevier Saunders; 2013:chap P.

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Review Date 4/21/2015

Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

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