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Test ID: PKU Phenylalanine and Tyrosine, Plasma

Reporting Name

Phenylalanine and Tyrosine, P

Useful For

Monitoring effectiveness of dietary therapy in patients with hyperphenylalaninemia

Specimen Type

Plasma


Specimen Required


Collection Container/Tube:

Preferred: Green top (sodium heparin)

Acceptable: Lavender top (EDTA)

Submission Container/Tube: Plastic vial

Specimen Volume: 0.5 mL

Collection Instructions: Fasting (4 hours or more in infants)

Additional Information:

1. Patient's age is required.

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


Specimen Minimum Volume

0.1 mL

Specimen Stability Information

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

Reference Values

PHENYLALANINE

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 60.6=result in nmol/mL

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

 

TYROSINE

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 55.6=result in nmol/mL

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

See Inborn Errors of Amino Acid Metabolism in Special Instructions.

Day(s) and Time(s) Performed

Monday through Friday; 10 a.m.

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 U.S. Food and Drug Administration.

CPT Code Information

84030-Phenylalanine

84510-Tyrosine

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

Clinical Information

Phenylketonuria (PKU) is the most frequent inherited disorder of amino acid metabolism (about 1:10,000-1:15,000) and was the first successfully treated inborn error of metabolism. It is inherited in an autosomal recessive manner and is caused by a defect in the enzyme phenylalanine hydroxylase (PAH), which converts the essential amino acid phenylalanine to tyrosine. Deficiency of PAH results in decreased levels of tyrosine and an accumulation of phenylalanine in blood and tissues. Untreated, PKU leads to severe brain damage with intellectual impairment, behavior abnormalities, seizures, and spasticity. The level of enzyme activity differentiates classic PKU (PAH activity <1%) from other milder forms; however, all are characterized by increased levels of phenylalanine (hyperphenylalaninemia). Treatment includes the early introduction of a diet low in phenylalanine.

 

Tetrahydrobiopterin (BH4) is a cofactor of not only PAH, but also of the tyrosine and tryptophan hydroxylases. Approximately 2% of patients with hyperphenylalaninemia have a deficiency of BH4, which causes a secondary deficit of the neurotransmitters dopamine and serotonin. There are 4 autosomal-recessive disorders associated with BH4 deficiency plus hyperphenylalaninemia; guanosine triphosphate cyclohydrolase deficiency, 6-pyruvoyl tetrahydropterine synthase deficiency, dihydropteridine reductase deficiency, and pterin-4 alpha carbinolamine dehydratase (PCD) deficiency. This group of disorders, with the exception of PCD, is characterized by progressive dystonia, truncal hypotonia, extremity hypertonia, seizures, and mental retardation though milder presentations exist. PCD has no symptoms other than transient alterations in tone. Treatment may include administration of BH4, L-dopa (and carbidopa) 5-hydroxytryptophan supplements, and a low phenylalanine diet.

 

Tyrosine is a nonessential amino acid that derives from dietary sources, the hydroxylation of phenylalanine, or protein breakdown. Primary (PKU) and secondary (defects of BH4 metabolism) hyperphenylalaninemia can cause abnormally low levels of tyrosine. Measurement of the phenylalanine:tyrosine ratio is helpful in monitoring appropriate dietary intake.

Interpretation

The quantitative results of phenylalanine and tyrosine with age-dependent reference values are reported without added interpretation. When applicable, reports of abnormal results may contain an interpretation based on available clinical interpretation.

 

A phenylalanine:tyrosine ratio higher than 3 is considered abnormal.

Clinical Reference

1. Mitchell GA, Grompe M, Lambert M, Tanguay RM: Hypertyrosinemia. In The Online Metabolic and Molecular Bases of Inherited Diseases. Edited by D Valle, AL BeaudetL, B Vogelstein, ET AL. New York, McGraw-Hill Medical, 2014. Accessed June 30, 2015. Available at http://ommbid.mhmedical.com/content.aspx?bookid=971&Sectionid=62673883

2. Donlon J, Sarkissian C, Levy H, Scriver CR: Hyperphenylalaninemia: Phenylalanine hydroxylase deficiency. In The Online Metabolic and Molecular Bases of Inherited Diseases. Edited by D Valle, AL Beaudet, B Vogelstein, et al. New York, McGraw-Hill Medical, 2014. Accessed June 30, 2015. Available at http://ommbid.mhmedical.com/content.aspx?bookid=971&Sectionid=62673211. Accessed June 30, 2015

3. Burgard P, Luo X, Hoffmann GF: Phenylketonuria. In Pediatric Endocrinology and Inborn Errors of Metabolism. Edited by K Sarafoglou, GF Hoffmann, KS Roth. New York, McGraw-Hill Medical Division, 2009, pp 163-168

4. Blau N, Thony B: Hyperphenylalanemias: Disorders of tetrahydrobiopterin metabolism. In Pediatric Endocrinology and Inborn Errors of Metabolism. Edited by K Sarafoglou, GF Hoffmann, KS Roth. New York, McGraw-Hill Medical Division, 2009, pp 169-175

Analytic Time

3 days (not reported on Saturday or Sunday)

Method Name

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

Forms

New York Clients-Informed consent is required. Please document on the request form or electronic order that a copy is on file. An Informed Consent for Genetic Testing (T576) is available in Special Instructions.

Mayo Medical Laboratories | Genetics and Pharmacogenomics Catalog Additional Information:

mml-biochemical