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26097 Phenylalanine and Tyrosine, Plasma (PKU)

Phenylalanine and Tyrosine, Plasma (PKU)
Test Code: PKUSO
Synonyms/Keywords

Phenylalanine Includes Tyrosine
PKU (Phenylketonuria)
Tyrosine
Tyrosinemia
Hyperphenylalaninemia

Test Components
Phenylalanine and Tyrosine
Useful For
Monitoring effectiveness of dietary therapy in patients with hyperphenylalaninemia
Specimen Requirements
Fasting Required Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
​Yes (4 hours or more in infants) ​Plasma ​Sodium Heparin Green Top Tube (GTT) ​EDTA Lavender Top Tube (LTT) ​0.5 mL ​0.1 mL
Collection Processing Instructions

​Patient's age is required

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

Specimen Stability Information
Specimen Type Temperature Time
​Plasma ​ ​Frozen(preferred) ​14 days
​Refrigerate ​14 days
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Mayo Clinic Laboratories ​Monday through Friday ​3 days (No results on Saturday or Sunday) ​Liquid Chromatography-Tandem Mass Spectrometer (LC-MS/MS)
Reference Lab
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.

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 Range Information
Performing Location Reference Range
​Mayo Clinic Laboratories

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

> or =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

 

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

> or =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

 

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.

Outreach CPTs
CPT Modifier
(if needed)
Quantity Description Comments
​84030 ​1 ​Phenylalanine
​84510 ​1 ​Tyrosine
Classification
This test was developed and its performance characteristics determined by Mayo Medical Laboratories.
Synonyms/Keywords

Phenylalanine Includes Tyrosine
PKU (Phenylketonuria)
Tyrosine
Tyrosinemia
Hyperphenylalaninemia

Test Components
Phenylalanine and Tyrosine
Ordering Applications
Ordering Application Description
​Centricity ​Phenylalanine and Tyrosine, Plasma (PKU)
​Cerner ​Phenylalanine and Tyrosine, Plasma (PKU)
​COM ​Phenylalanine and Tyrosine, Plasma (PKU)
If the ordering application you are looking for is not listed, contact your local laboratory for assistance.
Specimen Requirements
Fasting Required Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
​Yes (4 hours or more in infants) ​Plasma ​Sodium Heparin Green Top Tube (GTT) ​EDTA Lavender Top Tube (LTT) ​0.5 mL ​0.1 mL
Collection Processing

​Patient's age is required

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

Specimen Stability Information
Specimen Type Temperature Time
​Plasma ​ ​Frozen(preferred) ​14 days
​Refrigerate ​14 days
Useful For
Monitoring effectiveness of dietary therapy in patients with hyperphenylalaninemia
Test Components
Phenylalanine and Tyrosine
Reference Range Information
Performing Location Reference Range
​Mayo Clinic Laboratories

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

> or =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

 

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

> or =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

 

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.

For more information visit:
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Mayo Clinic Laboratories ​Monday through Friday ​3 days (No results on Saturday or Sunday) ​Liquid Chromatography-Tandem Mass Spectrometer (LC-MS/MS)
Reference Lab
For billing questions, see Contacts
Outreach CPTs
CPT Modifier
(if needed)
Quantity Description Comments
​84030 ​1 ​Phenylalanine
​84510 ​1 ​Tyrosine
Classification
This test was developed and its performance characteristics determined by Mayo Medical Laboratories.
For most current information refer to the Marshfield Laboratory online reference manual.