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Serum Phenobarbital Concentration (Mass Concentration)

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

This test measures the amount of phenobarbital, a medication used to control seizures, in your blood. It helps doctors check if your dose is working properly and at safe levels.

Quick summary

["Phenobarbital levels help ensure your seizure control is effective without excessive side effects."]

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

Key facts at a glance

LOINC
10547-8
Short code
Primidone+Phenobarb SerPl-mCnc
Synonyms
Primidone+Phenobarb SerPl-mCnc
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 Is This Test Done?

Your doctor may order this test if you’re taking phenobarbital to manage epilepsy or other seizure disorders. It helps monitor whether your medication is at an appropriate level to prevent seizures while minimizing risks like drowsiness or long-term side effects.

What to Expect Before the Test

No special preparation is needed. You’ll provide a blood sample, usually drawn from a vein in your arm. The process takes a few minutes, and you may feel a slight pinch. If you’re on blood thinners, let your doctor know before the test.

What the Results Mean

Results are compared to a general reference range for phenobarbital, but individual ranges may vary based on your health and medication. High levels might indicate overdose or poor metabolism, while low levels could mean your dose isn’t sufficient. Your doctor will interpret results in the context of your overall health and treatment plan.

Common Reasons for Testing

Doctors may test phenobarbital levels if you experience unexplained seizures, side effects (like excessive drowsiness), or if your medication isn’t controlling seizures as expected. It’s also used to adjust doses over time.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

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. Typically, it’s checked periodically (e.g., every few months) to adjust doses if needed, especially if you’re taking other medications that interact with phenobarbital.
Can I eat or drink before this test?
Yes, you can eat and drink normally before the test. Avoiding food or fluids isn’t required unless your doctor specifies otherwise.
What are the possible side effects of phenobarbital?
Common side effects include drowsiness, dizziness, or fatigue. Rarely, long-term use may affect bone density or liver function. Always discuss side effects with your doctor, as they can sometimes be managed with dose adjustments or alternative medications.
  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