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Phenylalanine - Newborn Screening

Published: July 27, 2026Updated: August 25, 2026

Phenylalanine is an amino acid that is measured in newborns to detect phenylketonuria (PKU), a genetic disorder that can lead to intellectual disability if left untreated.

Quick summary

Phenylalanine levels are screened in newborns to identify phenylketonuria (PKU).

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
mg/dL
Clinical utility
screening
Result kind
quantitative
FDA-cleared
FDA Cleared General Assay
interpretation: {"text":"Phenylalanine levels between 0–4 mg/dL are considered normal and indicate a low risk of PKU.","label":"Normal"}; {"text":"Levels above 4 mg/dL may indicate a potential risk of PKU and warrant further diagnostic testing.","label":"Elevated"}

Phenylalanine is an essential amino acid that plays a crucial role in the body's protein synthesis and other metabolic processes. In newborn screening, the levels of phenylalanine are measured to identify potential cases of phenylketonuria (PKU), a rare but serious genetic disorder.

PKU is caused by a deficiency of the enzyme phenylalanine hydroxylase, which results in the accumulation of phenylalanine in the blood and tissues. If not detected and treated early, it can lead to severe intellectual disability and other neurological problems.

The screening process typically involves collecting a blood sample from the newborn's heel within the first few days of life. Early detection and management of PKU, primarily through dietary modifications, can prevent the onset of symptoms and ensure normal development.

Frequently asked questions

Why is phenylalanine screening important for newborns?
Phenylalanine screening is crucial for early detection of PKU, which can prevent severe developmental issues if managed with a special diet.
What happens if my newborn's phenylalanine levels are high?
If phenylalanine levels are elevated, your healthcare provider will recommend further testing to confirm PKU and discuss dietary management options.
  1. 1
    Phenylketonuria (PKU) - Genetic and Rare Diseases Information CenterSource
  2. 2
    Newborn Screening - Centers for Disease Control and PreventionSource
  3. 3
    Phenylalanine - MedlinePlusSource

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 25, 2026. · Read our editorial policy