Phenylalanine Level — Newborn Screening (Infant Fed Other Than Breast Milk)
A newborn screening test that measures phenylalanine in blood to help detect phenylketonuria (PKU) and related metabolic conditions.
["An elevated phenylalanine level may signal phenylketonuria and should be confirmed promptly."]
Key facts at a glance
- LOINC
- 61432-1
- Short code
- Age fed other than breastmlk PhenX
- Synonyms
- Age fed other than breastmlk PhenX
- Category
- liver
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 10
- Ref high
- 40
- Critical range
- — – 500 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- phenx
- CPT
- 84484
- SNOMED
- 302774002
- 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 in a baby's blood. It is commonly included in newborn screening because high levels can occur with phenylketonuria (PKU), an inherited condition in which the body cannot properly process phenylalanine.
Why might I need this test?
Newborn screening can identify infants with PKU before symptoms develop. Early detection allows treatment to begin quickly, which can help prevent serious neurological and developmental problems.
How is the test performed?
A small blood sample is collected, often by a heel prick for newborn screening. A laboratory measures the phenylalanine level and may repeat or confirm an unexpected result with another blood test.
How do I prepare for the test?
No special preparation is usually required. The result is interpreted according to the baby's age, feeding history, health, and screening method, so parents should tell the healthcare team whether the infant has been fed breast milk, formula, or both.
Reference range
Adult: 10-40 U/LN/A
No special preparation required. Avoid fatty meals before the test if possible.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
A higher-than-expected phenylalanine level may indicate PKU or another condition that affects phenylalanine metabolism, but an initial screening result is not diagnostic by itself. Results can also vary with feeding, prematurity, illness, and the timing of the sample, so an abnormal screen may require repeat or confirmatory testing.
Short name
Phenylalanine newborn screen
Frequently asked questions
What does the phenylalanine newborn screen measure?
Why is feeding history included with this test?
What happens if the result is abnormal?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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 17, 2026. · Read our editorial policy