Phenylalanine in Serum (or Plasma) – Qualitative Screening
This test checks whether phenylalanine levels in your blood are unusually high, which can occur due to genetic conditions like phenylketonuria (PKU). Early detection helps manage health risks during pregnancy.
["High phenylalanine may harm your baby’s brain development if untreated."]
Key facts at a glance
- LOINC
- 65870-8
- Short code
- Things during pregnancy ever PhenX
- Synonyms
- Things during pregnancy ever PhenX
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Critical range
- 0.5 N/A – —
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- phenx
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
Why This Test Matters During Pregnancy
Phenylalanine is an essential amino acid your body needs to build proteins. In some people, a genetic disorder called phenylketonuria (PKU) prevents the body from breaking down phenylalanine properly. If phenylalanine builds up in the blood, it can cause serious health problems for both you and your baby. This test helps identify if phenylalanine levels are too high.
How the Test Works
A small blood sample is taken from your arm. The lab tests the sample for phenylalanine using a method called **immunoassay (IA)** or **nephelometry (neph)**, which detects the substance’s presence or concentration. Results are reported as either **qualitative (positive/negative)** or, if quantified, as a mass concentration (e.g., mg/dL).
What to Expect Before the Test
No special preparation is needed, but you may want to drink plenty of water the day of the test to help your blood flow smoothly. Wear comfortable clothing and bring any medications you’re currently taking, as some may affect results.
What to Expect After the Test
You’ll likely feel minor discomfort from the needle stick, but most people recover quickly. If you experience excessive bruising or pain, contact your healthcare provider. Follow-up care depends on the test results—your doctor will guide you next steps.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance
Short name
Phenylalanine Screening
Frequently asked questions
If your test shows high phenylalanine levels, your doctor may recommend dietary changes, supplements, or monitoring to prevent complications. Untreated high phenylalanine can lead to intellectual disabilities, developmental delays, or other health issues in your baby.
How often will I need this test?
What if my test results are normal?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy