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 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, 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

Konczal LL, Zinn AB. Inborn errors of metabolism. In: Martin RJ, Fanaroff AA, Walsh MC, eds. Fanaroff and Martin's Neonatal-Perinatal Medicine. 11th ed. Philadelphia, PA: Elsevier; 2020:chap 90.

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.