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Phenobarbital Drug Concentration (Serum/Plasma)

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 adjust it as needed.

Quick summary

["This test ensures your seizure medication is at the right level to manage symptoms safely."]

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

Key facts at a glance

LOINC
73209-9
Short code
Phenobarb ind plt IgG SerPl Ql FC
Synonyms
Phenobarb ind plt IgG SerPl Ql FC
Category
hematology
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
flow-cytometry
CPT
82784
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

Phenobarbital is a long-acting anticonvulsant that works by stabilizing nerve cells in the brain. Blood levels help prevent seizures while minimizing side effects like drowsiness or liver issues. Too little may mean seizures aren’t controlled, while too much could cause toxicity.

How the Test Works

A small blood sample is drawn, usually from your arm, and sent to a lab. The lab uses a sensitive immunoassay (or similar method) to detect phenobarbital in your serum or plasma.

What to Expect Before the Test

No special preparation is needed, but inform your doctor about any other medications you’re taking, as they may affect results. Avoid eating or drinking right before the draw if instructed.

Possible Results & Next Steps

Results are typically reported in micrograms per milliliter (mcg/mL). Your doctor will compare these numbers to your specific dose and symptoms to decide if adjustments are needed. Follow-up tests may be scheduled if levels are outside the target range.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Phenobarbital Level

Frequently asked questions

What does a high phenobarbital level mean?
High levels may indicate overdose, liver toxicity, or improper dosing. Your doctor might reduce your medication or monitor you closely for side effects like confusion, slow breathing, or liver pain.
What does a low phenobarbital level mean?
Low levels could mean your seizures aren’t controlled, especially if you’re taking a lower dose than prescribed. Your doctor may increase your medication or switch to a different anticonvulsant.
How often should I get this test?
Your doctor will determine how often you need testing based on your condition, medication regimen, and how well you’re responding. Regular monitoring helps balance safety and effectiveness.
  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