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Test ID PKU Phenylalanine and Tyrosine, Plasma

Useful For

Monitoring effectiveness of dietary therapy in patients with hyperphenylalaninemia

Genetics Test Information

Defects in phenylalanine hydroxylase (PAH) cause the majority of cases of hyperphenylalaninemia (HPA); however, approximately 2% of infants with HPA have impaired synthesis or recycling of tetrahydrobiopterin (BH4).


Phenylketonuria: Evaluation of patients with hyperphenylalaninemia or monitoring effectiveness of dietary therapy. This test is not sufficient follow-up for abnormal newborn screening results, because other causes of hyperphenylalaninemia (eg, BH4 deficiency) cannot be excluded by this test alone.


Tyrosinemia, type I: For medical management.

Method Name

Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Reporting Name

Phenylalanine and Tyrosine, P

Specimen Type


Necessary Information

1. Patient's age is required.

2. Include family history, clinical condition (asymptomatic or acute episode), diet, and drug therapy information.

Specimen Required

Patient Preparation: Fasting (4 hours or more for infants)

Collection Container/Tube:

Preferred: Green top (sodium heparin)

Acceptable: Lavender top (EDTA)

Submission Container/Tube: Plastic vial

Specimen Volume: 0.5 mL

Specimen Minimum Volume

0.1 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Plasma Frozen (preferred) 14 days
  Refrigerated  14 days

Reference Values


Premature: 98-213 nmol/mL

0-31 days: 38-137 nmol/mL

1-24 months: 31-75 nmol/mL

2-18 years: 26-91 nmol/mL

≥19 years: 35-85 nmol/mL


Conversion Formulas:

Result in mg/dL x 60.5=result in nmol/mL

Result in nmol/mL x 0.0165=result in mg/dL



Premature: 147-420 nmol/mL

0-31 days: 55-147 nmol/mL

1-24 months: 22-108 nmol/mL

2-18 years: 24-115 nmol/mL

≥19 years: 34-112 nmol/mL


Conversion Formulas:

Result in mg/dL x 55.2=result in nmol/mL

Result in nmol/mL x 0.0181=result in mg/dL

Day(s) Performed

Monday through Friday

Performing Laboratory

Mayo Clinic Laboratories in Rochester

CPT Code Information



LOINC Code Information

Test ID Test Order Name Order LOINC Value
PKU Phenylalanine and Tyrosine, P In Process


Result ID Test Result Name Result LOINC Value
8380 Phenylalanine, P 14875-9
8627 Tyrosine, P 20660-7

Test Classification

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

Report Available

2 to 4 days

NY State Approved