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Phenobarbital and Primidone Serum Concentration (Substance Concentration)

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

This test measures the amounts of two medications—phenobarbital and primidone—in your blood. These drugs are used to control seizures, and your levels are checked to ensure they’re working safely.

Quick summary

["Your doctor may order this test to check if your seizure medication is at the right dose for you."]

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

Key facts at a glance

LOINC
34365-7
Short code
Primidone+Phenobarb SerPl-sCnc
Synonyms
Primidone+Phenobarb SerPl-sCnc
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 recommend this test if you’re taking phenobarbital or primidone to manage epilepsy. They might also order it if you have side effects like drowsiness, dizziness, or trouble concentrating. This helps them adjust your dose to keep your medication levels safe and effective.

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 blood. It usually takes a few minutes. If you’re on blood thinners, let your provider know before the test.

What the Results Mean

Your results show how much of each medication is in your blood at the time of testing. While there are no strict reference ranges for these drugs, your provider will compare them to your typical therapeutic range to guide dosing adjustments.

Possible Side Effects of Medication

High or low levels of phenobarbital or primidone can cause side effects like drowsiness, confusion, or slowed breathing. If you experience these symptoms, contact your doctor right away.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Phenobarbital & Primidone Levels

Frequently asked questions

What does it measure?
This test measures the concentration of phenobarbital and primidone in your blood. These medications are used to prevent seizures and can affect how other drugs work in your body. Keeping levels in the right range helps prevent both too much and too little medication.
Why is it important to monitor these levels?
Monitoring these levels helps ensure your medication is working properly without causing harmful side effects. For example, too much of these drugs can slow your breathing or make you very sleepy, while too little may not control your seizures effectively.
Can diet or other medications affect these test results?
Yes, certain foods, other medications (including over-the-counter drugs), and even some herbal supplements can change how phenobarbital and primidone work in your body. Always tell your doctor about all medications and supplements you take before your test.
  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