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Phenytoin (Phenobarbital) Serum Concentration, Quantitative

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

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

Quick summary

["Phenytoin levels help ensure your medication dose is appropriate for your condition and side effects are minimized."]

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

Key facts at a glance

LOINC
74912-7
Short code
Primidone Trough SerPl-mCnc
Synonyms
Primidone Trough 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 This Test Is Done

Your healthcare provider may order this test if you’re taking phenytoin (brand names like Dilantin) to manage epilepsy or other seizure disorders. Regular monitoring helps prevent toxicity or inadequate dosing.

How the Test Works

A blood sample is taken from a vein, usually in your arm. The lab uses a sensitive method to detect phenytoin in your serum, providing a precise measurement of its concentration.

What to Expect Before the Test

No special preparation is needed, but inform your provider if you’re taking other medications, have recent injuries, or are pregnant. Avoid eating or drinking for 8–12 hours before the test if instructed.

What to Expect During the Test

You’ll lie down comfortably while a nurse inserts a small needle into your vein. The blood sample will be drawn quickly, and you may feel a brief pinch. The process takes only a few minutes.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Phenytoin Level

Frequently asked questions

How often should I get this test?
Your provider will determine the timing based on your medication regimen and symptoms. Typically, tests are done 12 hours after taking your dose to measure the trough level (lowest concentration).
What does a ‘normal’ result mean?
There’s no single ‘normal’ range for phenytoin, as it varies by individual and dose. Your results will be compared to your baseline and adjusted with your healthcare team.
Can I eat or drink before the test?
Follow your provider’s instructions, but generally, avoid food/drink for 8–12 hours before the test to ensure accurate results.
  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