Phenylalanine and Tyrosine, Plasma
Use
This test is useful for monitoring the effectiveness of dietary therapy in patients with hyperphenylalaninemia, a condition often caused by defects in phenylalanine hydroxylase (PAH). While it helps in the evaluation of patients with hyperphenylalaninemia or phenylketonuria (PKU), it is not sufficient as a follow-up for abnormal newborn screening results since other causes of HPA, like BH4 deficiency, cannot be excluded. Tyrosinemia type I requires medical management, and this test assists in such scenarios by providing measurements of phenylalanine and tyrosine levels as well as their ratios.
Special Instructions
Patient preparation includes fasting for 8 hours; infants should be sampled before the next feeding. Complete the Biochemical Genetics Test Request form (T798) if not ordering electronically.
Limitations
This test is not sufficient to establish a diagnosis of hyperphenylalaninemia on its own, as it cannot exclude all other causes, such as BH4 deficiency. Improper specimen collection and handling, along with inappropriate test selection, may also affect diagnostic accuracy.
New York Approved
Methodology
Mass Spectrometry (LC-MS/MS)
Biomarkers
LOINC Codes
- 101402-6
- 14875-9
- 20660-7
Result Turnaround Time
2-4 days
Related Documents
For more information, please review the documents below
Specimen
Plasma
Volume
0.5 mL
Minimum Volume
0.1 mL
Container
Plastic vial
Collection Instructions
Centrifuge and aliquot plasma into a plastic vial. Send plasma frozen.
Patient Preparation
Fasting: 8 hours, required; Infants should have specimen collected before next feeding
Causes for Rejection
Gross hemolysis: OK, Gross lipemia: OK, Gross icterus: OK
Stability Requirements
| Temperature | Period |
|---|---|
| Refrigerated | 14 days |
| Frozen | 14 days |