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Pharmacokinetic measurement of phenobarbital (serum 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 your healthcare provider check if your dose is working properly.

Quick summary

["This test helps ensure your medication level stays safe and effective for managing seizures."]

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

Key facts at a glance

LOINC
67758-3
Short code
HEDIS 2012-13 MPM-E PHENobarbital
Synonyms
HEDIS 2012-13 MPM-E PHENobarbital
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 doctor may order this test if you’re taking phenobarbital to treat epilepsy or other seizure disorders. It helps monitor whether your medication is at an appropriate level to control symptoms without causing excessive side effects.

What to Expect Before the Test

No special preparation is needed. You’ll have a small needle inserted into your arm or vein to draw a blood sample. This is typically done in a clinic or lab setting.

What the Test Results Mean

Results are used to adjust your medication dose if needed. Low levels may indicate the drug isn’t working well, while high levels could signal toxicity. Your doctor will discuss findings with you.

Common Reasons for Testing

This test is often done during follow-up visits, after a change in medication, or if you experience unusual side effects like drowsiness, confusion, or slow breathing.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Phenobarbital is a long-acting anticonvulsant. Blood levels help prevent seizures while minimizing risks like sedation or respiratory depression. Abnormal levels may require dose adjustments or alternative medications.

Short name

Phenobarbital Level

Frequently asked questions

How long does it take to get results?
Results are usually available within 1–3 business days, depending on your lab’s processing time.
What if my results are outside the expected range?
Your doctor will review the results with you and may adjust your medication dose, especially if levels are too high or too low. They’ll determine the best course of action based on your overall health and symptoms.
Can I eat or drink before this test?
You can eat and drink normally before the test, as it’s a blood draw with no dietary restrictions.
  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