Skip to content

Phenobarbital (Serum/Plasma Concentration)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of phenobarbital, a medication used to control seizures, in your blood.

Quick summary

["Knowing your phenobarbital level helps doctors manage your seizure control safely."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
74252-8
Short code
HEDIS 2014 Phenobarb
Synonyms
HEDIS 2014 Phenobarb
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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 Is Done

Your healthcare provider may order this test if you’re taking phenobarbital to treat epilepsy or other seizure disorders. It helps ensure your medication levels are within a safe range to prevent seizures or side effects.

How the Test Works

A blood sample is taken from a vein in your arm. The lab uses a sensitive test to measure phenobarbital in your serum or plasma.

What to Expect Before the Test

No special preparation is needed, but inform your doctor about all medications you’re taking, including over-the-counter drugs or supplements, as they may affect results.

What the Results Mean

Results help doctors balance your medication dose to avoid either too little (which may lead to seizures) or too much (which could cause drowsiness or other side effects).

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Phenobarbital levels are critical for managing epilepsy. Too low may mean seizures aren’t controlled, while too high can cause excessive sedation or liver strain. Regular monitoring helps prevent these risks.

Short name

Phenobarbital Level

Frequently asked questions

How often should I get this test?
Your doctor will determine the frequency based on your treatment plan, but it’s often checked every few weeks or months to adjust your dose if needed.
What happens if my levels are too high or too low?
If levels are too high, your doctor may reduce your dose or temporarily stop the medication to avoid side effects. If levels are too low, they may increase your dose or add another medication to better control seizures.
Can I drink alcohol before this test?
No, avoid alcohol before the test as it can interfere with blood test results and may affect how phenobarbital works in your body.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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 16, 2026. · Read our editorial policy