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Test ID: PKUBS Phenylalanine and Tyrosine, Blood Spot


Specimen Required


Supplies: Card-Blood Spot Collection (Filter Paper) (T493)

Preferred: Blood Spot Collection Card (T493)

Acceptable: Whatman Protein Saver 903 Paper, Ahlstrom 226 filter paper

Specimen Volume: 2 Blood spots

Collection Instructions:

1. Let blood dry on filter paper at room temperature in a horizontal position for 3 or more hours.

2. At least 2 spots should be complete, ie, unpunched.

3. Do not expose specimen to heat or direct sunlight.

4. Do not stack wet specimens.

5. Keep specimen dry.

Additional Information:

1. Dried blood spots collected with EDTA, sodium heparin, lithium heparin, or ACD containing devices are acceptable.

2. For collection instructions in Spanish, see Blood Spot Collection Card-Spanish Instructions (T777) in Special Instructions.

3. For collection instructions in Chinese, see Blood Spot Collection Card-Chinese Instructions (T800) in Special Instructions.


Useful For

Monitoring effectiveness of dietary therapy in patients with hyperphenylalaninemia

Method Name

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

Reporting Name

Phenylalanine and Tyrosine, BS

Specimen Type

Whole blood

Specimen Minimum Volume

Blood Spots: 1

Specimen Stability Information

Specimen Type Temperature Time
Whole blood Ambient (preferred) 21 days
  Frozen  10 days
  Refrigerated  10 days

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.

Reference Values

PHENYLALANINE

29-95 nmol/mL

TYROSINE

29-149 nmol/mL

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 information.

 

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 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=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

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

Day(s) and Time(s) Performed

Monday through Friday; 1 p.m.

Analytic Time

3 days

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

Phenylalanine-84030

Tyrosine-84510

LOINC Code Information

Test ID Test Order Name Order LOINC Value
PKUBS Phenylalanine and Tyrosine, BS In Process

 

Result ID Test Result Name Result LOINC Value
92405 Phenylalanine, BS In Process
92406 Tyrosine, BS In Process
92407 Reviewed By In Process
Mayo Medical Laboratories | Genetics and Pharmacogenomics Catalog Additional Information:

mml-biochemical