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Phenylalanine (PKU) Newborn Screening Test

Published: August 9, 2026Updated: August 16, 2026

Measures phenylalanine, usually in a newborn blood spot, to screen for phenylketonuria (PKU).

Quick summary

["An abnormal newborn phenylalanine screen may indicate phenylketonuria and should be followed by confirmatory testing."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
61549-2
Short code
Have milk drink or cereal in 30D PhenX
Synonyms
Have milk drink or cereal in 30D PhenX
Category
liver
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
phenx
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What is this test?

This test measures the amount of phenylalanine, an amino acid, in a blood sample. It is commonly part of newborn screening and is usually performed using a dried blood spot collected by a heel prick.

Why might I need this test?

Newborn screening can identify infants with high phenylalanine levels before symptoms develop. Early detection and treatment of phenylketonuria (PKU) help prevent intellectual disability and other health problems.

How is the test performed?

A few drops of blood are placed on filter paper and sent to a laboratory for analysis. If the result is abnormal, the infant will usually need a repeat or confirmatory blood test.

How do I prepare for the test?

The baby generally needs to have received breast milk, formula, or cereal for the period specified by the screening program before collection. Follow the laboratory or newborn-screening staff's instructions, especially if the infant was premature, ill, or hospitalized.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

An elevated phenylalanine result may suggest PKU, another cause of hyperphenylalaninemia, or a temporary increase related to illness or feeding. Screening results are not diagnostic, so an abnormal result requires confirmatory testing and assessment by the infant's clinician. Early treatment with an appropriate diet and, when needed, medication can prevent complications.

Short name

Phenylalanine screen

Frequently asked questions

Is this the same as testing for a newborn's thyroid function?
No. This test specifically measures phenylalanine to screen for PKU and related conditions.
Does the baby need to be eating normally before the blood sample is collected?
Usually, the infant should have been receiving breast milk, formula, or cereal for the required period. The screening program or laboratory should provide the exact instructions, particularly for premature or hospitalized babies.
What happens if the screening result is abnormal?
The result is usually repeated or confirmed with another blood test. The clinician may also review the infant's diet, medical history, and genetic or other laboratory findings.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy