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Test ID: PKUSC Phenylalanine and Tyrosine, Self-Collect, Blood Spot

Ordering Guidance

For follow-up of an abnormal newborn screen for potential phenylketonuria, order PKU / Phenylalanine and Tyrosine, Plasma

Necessary Information

1. Patient's age is required.

2. Patient's street address, city, state, ZIP (postal) code, country, and home phone are required (post-office [PO] boxes are not acceptable delivery locations).

Specimen Required

Supplies: Blood Spot Collection-Self Collect (T858)

Container/Tube: Blood Spot Self Collection Card

Specimen Volume: 2 Blood spots

Additional Information:

1. Order test each time the patient is to collect a dried blood specimen at home and mail the specimen directly to Mayo Clinic Laboratories.

2. Order should be placed a minimum of 3 days prior to desired date of collection.

3. Enter patient's address information for each order created, including street address (post office [PO] boxes are not acceptable delivery locations), city, state abbreviation, zip code, country, and home phone number.

4. For each order, the Blood Spot Collection-Self Collect kit will be mailed directly to the patient for self-collection (delivery to a PO box will not occur).

5. For more information on how to collect blood spots, see the following:

-How to Collect Dried Blood Spot Samples via fingerstick.

-Blood Spot Collection Instructions-Fingerstick

-Blood Spot Collection Instructions-Fingerstick-Spanish

Useful For

Monitoring effectiveness of therapy in patients with hyperphenylalaninemia in a patient-collected specimen


This test is not sufficient for follow-up for abnormal newborn screening results or for establishing a diagnosis of a specific cause of hyperphenylalaninemia.

Method Name

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

Reporting Name

Phenylalanine and Tyrosine, SC, BS

Specimen Type

Whole blood

Specimen Minimum Volume

1 Blood spot

Specimen Stability Information

Specimen Type Temperature Time Special Container
Whole blood Ambient (preferred) 21 days FILTER PAPER
  Frozen  10 days FILTER PAPER
  Refrigerated  10 days FILTER PAPER

Clinical Information

Phenylketonuria (PKU) is the most frequent inherited disorder of amino acid metabolism (occurring in about 1:10,000-1:15,000 births) 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. Some patients may also benefit from adjuvant tetrahydrobiopterin (BH4) supplementation (a cofactor for PAH) or enzyme substitution therapy.


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 tetrahydropterin synthase deficiency, dihydropteridine reductase deficiency, and pterin-4 alpha carbinolamine dehydratase (PCD) deficiency. This group of disorders, apart from PCD, is characterized by progressive dystonia, truncal hypotonia, extremity hypertonia, seizures, and intellectual disability 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 is derived 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


27.0-107.0 nmol/mL



<4 weeks: 40.0-280.0 nmol/mL

≥4 weeks: 25.0-150.0 nmol/mL


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: Valle DL, Antonarakis S, Ballabio A, Beaudet AL, Mitchell GA, eds. The Online Metabolic and Molecular Bases of Inherited Disease. McGraw Hill; 2019. Accessed February 8, 2023. Available at

2. Donlon J, Sarkissian C, Levy H, Scriver CR: Hyperphenylalaninemia: Phenylalanine hydroxylase deficiency. In: Valle DL, Antonarakis S, Ballabio A, Beaudet AL, Mitchell GA, eds. The Online Metabolic and Molecular Bases of Inherited Disease. McGraw-Hill; 2019. Accessed February 8, 2023. Available at

3. Regier DS, Greene CL: Phenylalanine hydroxylase deficiency. In: Adam MP, Ardinger HH, Pagon RA, et al, eds. GeneReviews [Internet]. University of Washington, Seattle; 2000. Updated January 5, 2017. Accessed February 8, 2023. Available at

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.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
PKUSC Phenylalanine and Tyrosine, SC, BS 79621-9


Result ID Test Result Name Result LOINC Value
610515 Tyrosine, BS 35571-9
610516 Phenylalanine, BS 29573-3
610514 Reviewed By 18771-6
BG735 Patient Street Address (No PO Box) 56799-0
BG736 Patient City 68997-6
BG737 Patient State 46499-0
BG738 Patient Zip Code 45401-7
BG742 Patient Country 77983-5
BG739 Patient Home Phone 42077-8

Day(s) Performed

Monday through Friday

Report Available

3 to 5 days
Mayo Clinic Laboratories | Genetics and Pharmacogenomics Catalog Additional Information: