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Phenylalanine (Phe)

SI UNITS (recommended)

CONVENTIONAL UNITS

(Essential Aromatic Amino Acid - Critical in Newborn Screening & Diagnosis of Phenylketonuria)

Synonyms

  • Phenylalanine
  • Phe
  • L-Phenylalanine
  • Essential aromatic amino acid
  • PKU screening analyte

Units of Measurement

  • µmol/L
  • mg/L
  • mg/dL
  • mg/100 mL
  • mg%
  • µg/mL

Molecular Weight

165.19 g/mol

Key Unit Conversions

Mass ↔ Molar

1 mg/L=6.05 µmol/L1\ \text{mg/L} = 6.05\ \text{µmol/L}1 mg/L=6.05 µmol/L 1 µmol/L=0.165 mg/L1\ \text{µmol/L} = 0.165\ \text{mg/L}1 µmol/L=0.165 mg/L

mg/dL ↔ mg/L

1 mg/dL=10 mg/L1\ \text{mg/dL} = 10\ \text{mg/L}1 mg/dL=10 mg/L

µg/mL

1 µg/mL=1 mg/L1\ \text{µg/mL} = 1\ \text{mg/L}1 µg/mL=1 mg/L

mg%

\text{mg%} = \text{mg/dL}

Description

Phenylalanine is an essential amino acid required for:

  • Protein synthesis
  • Neurotransmitter precursors (tyrosine → dopamine, norepinephrine, epinephrine)
  • Melanin synthesis
  • Thyroid hormone pathway

Phe levels rise markedly when phenylalanine hydroxylase (PAH) is deficient, causing:

Phenylketonuria (PKU)

A lifelong metabolic disorder leading to severe neurological damage if untreated.

Phe measurement is one of the core newborn screening tests worldwide.

Physiological Role

  • Essential component of dietary protein
  • Precursor to tyrosine, which is needed for:
    • Dopamine
    • Norepinephrine
    • Epinephrine
    • Melanin
    • Thyroid hormones (T3/T4)

Normal metabolism:

Phenylalanine→PAHTyrosine\text{Phenylalanine} \xrightarrow{\text{PAH}} \text{Tyrosine}PhenylalaninePAH​Tyrosine

Clinical Significance

HIGH Phenylalanine (Hyperphenylalaninemia)

1. Phenylketonuria (PKU)

Classic PKU → Phe > 1200 µmol/L
 Caused by:

  • PAH enzyme deficiency
  • Tetrahydrobiopterin (BH4) deficiency (cofactor)

2. Maternal PKU

High maternal Phe causes fetal:

  • Microcephaly
  • Heart defects
  • Growth restriction
  • Severe developmental delay

3. Liver Disease

Reduced PAH activity.

4. High-Protein Intake

Transient mild elevation.

5. Tetrahydrobiopterin (BH4) Deficiency

Leads to high Phe + neurotransmitter deficiency.

LOW Phenylalanine

Rare. Seen in:

  • Low protein intake
  • Severe malnutrition
  • Inborn errors of protein metabolism
  • Liver failure
  • Over-correction on PKU diet (too restrictive)

Low Phe can impair neurotransmitter synthesis.

Reference Intervals

Normal Serum Phenylalanine

  • 30 – 90 µmol/L
     (= 5 – 15 mg/L)

Newborn (screening)

  • < 120 µmol/L → normal
  • > 120 µmol/L → abnormal, repeat test
  • > 360 µmol/L → diagnostic of PKU
  • > 1200 µmol/L → classic PKU

Therapeutic Targets (PKU management)

  • < 360 µmol/L in infants & children
  • < 600 µmol/L in adults
  • < 300 µmol/L in pregnancy (maternal PKU)

Diagnostic Uses

1. Newborn Screening (Primary Use)

Detects PKU early → prevents irreversible brain injury.

2. Diagnosis of Hyperphenylalaninemia

Includes:

  • Classic PKU
  • Mild PKU
  • Variant forms
  • BH4 deficiency

3. Monitoring PKU Treatment

Diet therapy, BH4 therapy (sapropterin), pegvaliase.

4. Maternal PKU Management

Tight control of Phe during pregnancy.

5. Liver Disease Evaluation

Elevated Phe in hepatic dysfunction.

Analytical Notes

  • Sample: plasma or dried blood spot (DBS)
  • DBS used for newborn screening (tandem MS/MS)
  • Avoid hemolysis
  • Fasting improves accuracy (diet sensitive)
  • High tyrosine with high Phe suggests BH4 deficiency
  • Serial monitoring essential in PKU patients

Clinical Pearls

  • In PKU, untreated infants appear normal at birth - early screening is vital.
  • High Phe is toxic to the developing brain → causes severe intellectual disability if untreated.
  • Pegvaliase enzyme therapy is now used in adults with refractory high Phe.
  • PKU diets must maintain enough Phe for growth but low enough to prevent toxicity.
  • Maternal PKU requires strict control (<300 µmol/L) to prevent fetal damage.

Interesting Fact

Phenylalanine was the first metabolic disorder included in worldwide newborn screening programs, beginning with the historic Guthrie bacterial inhibition test.

References

  1. Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids & Metabolic Disorders
  2. ACMG Practice Guidelines - Diagnosis & Management of PKU
  3. Mayo Clinic Laboratories - Phenylalanine
  4. ARUP Consult - PKU & Hyperphenylalaninemia
  5. NIH / MedlinePlus - Phenylalanine Test

Last updated: January 27, 2026

Reviewed by : Medical Review Board

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