Phenylalanine and Tyrosine, Blood Spot
Use
This test is used for monitoring the effectiveness of therapy in patients with hyperphenylalaninemia. It is not sufficient for follow-up for abnormal newborn screening results or for establishing a diagnosis of a specific cause of hyperphenylalaninemia. Phenylketonuria (PKU) is the most frequent inherited disorder of amino acid metabolism, and its proper management includes monitoring phenylalanine and tyrosine levels. Patients with hyperphenylalaninemia need to be evaluated for adequate dietary management, and the phenylalanine:tyrosine ratio is crucial in assessing dietary intake.
Special Instructions
Patient's age is required. For follow-up of an abnormal newborn screen for potential phenylketonuria, order the plasma version of the test, PKU / Phenylalanine and Tyrosine, Plasma. Blood needs to be collected using specific types of filter paper or tubes containing EDTA and dried on filter paper. Detailed instructions for blood spot collection are available in multiple languages.
Limitations
This test does not provide sufficient follow-up for abnormal newborn screening results because other causes of hyperphenylalaninemia (such as tetrahydrobiopterin deficiency) cannot be excluded by this test alone. It is not useful for diagnosing the specific cause of hyperphenylalaninemia. Conditions that might result in specimen rejection include blood spot specimens that show serum rings or have multiple layers and insufficient specimen volume. Additionally, unapproved filter papers can cause specimen rejection.
New York Approved
Methodology
Mass Spectrometry (LC-MS/MS)
Biomarkers
LOINC Codes
- 79621-9
- 29573-3
- 35571-9
- 18771-6
Result Turnaround Time
3-5 days
Related Documents
For more information, please review the documents below
Specimen
Dried Blood Spot
Volume
2 Blood spots
Minimum Volume
1 Blood spot
Container
Blood Spot Collection Card
Collection Instructions
An alternative blood collection option for a patient older than 1 year is a fingerstick. Let blood dry on filter paper at room temperature in a horizontal position for a minimum of 3 hours. Do not expose specimen to heat or direct sunlight. Do not stack wet specimens. Keep specimen dry.
Causes for Rejection
Blood spot specimen that shows serum rings or has multiple layers; insufficient specimen; unapproved filter papers
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 90 days |
| Refrigerated | 90 days |
| Frozen | 90 days |