Definition

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.

Alternative Names

Phenylalanine - blood test; PKU - phenylalanine

How the Test is Performed

The test is most often 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, most often 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.

Newborn screening tests is a related article.

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 or lancet is inserted to draw blood, some infants feel moderate pain, while others feel only a prick or stinging sensation. The baby will most likely cry when the heel is pricked to get the blood sample. Studies have shown that babies whose mothers hold them skin-to-skin or breastfeed them during the procedure show less distress. Wrapping the baby tightly in a blanket, or offering a pacifier dipped in sugar water, may also help ease pain and calm the baby.

Why the Test is Performed

This test is done to screen infants for PKU, an uncommon 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.

Risks

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

References

Centers for Disease Control and Prevention website. About newborn screening. www.cdc.gov/newborn-screening/about/index.html. Updated December 17, 2024. Accessed April 29, 2025.

Kliegman RM, St. Geme JW, Blum NJ, et al. Defects in metabolism of amino acids. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 105.

Konczal LL. Inborn errors of metabolism. In: Martin RJ, Fanaroff AA, eds. Fanaroff and Martin's Neonatal-Perinatal Medicine. 12th ed. Philadelphia, PA: Elsevier; 2025:chap 94.

McPherson RA. Specific proteins. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 20.

Pasquali M, Longo N. Newborn screening and inborn errors of metabolism. In: Rifai N, Chiu RWK, Young I, Burnham Carey-Ann D, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. St Louis, MO: Elsevier; 2023:chap 60.