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Serum or plasma concentration of phenytoin (anticonvulsant drug)

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 is at an effective dose to prevent seizures while avoiding harmful side effects."]

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

Key facts at a glance

LOINC
3978-4
Short code
Primidone SerPl-mCnc
Synonyms
Primidone 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 to manage epilepsy or other seizure disorders. It helps monitor your drug levels to prevent seizures or overdose.

How the Test Works

A blood sample is taken, usually from a vein in your arm. The lab measures the concentration of phenytoin in your serum or plasma using a sensitive chemical test.

What to Expect Before the Test

No special preparation is needed, but inform your provider about any medications, supplements, or recent illnesses that could affect results.

Possible Results and Interpretation

Results are compared to a general reference range, but exact values may vary by lab. Low levels may indicate insufficient medication, while high levels could signal toxicity.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Phenytoin levels are critical for epilepsy management. Too little may lead to seizures, while too much can cause dizziness, nausea, or other serious side effects. Regular monitoring helps balance safety and effectiveness.

Short name

Phenytoin Level

Frequently asked questions

What causes phenytoin levels to change?
Several factors can affect phenytoin levels, including changes in dosage, liver function, kidney function, or other medications you’re taking. Stress, illness, or alcohol use may also impact absorption.
How often should I get this test?
Your provider will determine the frequency based on your treatment plan, but it’s often done every few weeks to months to adjust your dose as needed.
Can diet or supplements affect this test?
Some supplements, like St. John’s Wort, or certain foods (e.g., grapefruit) may interact with phenytoin. Always discuss all supplements or dietary changes with your healthcare team before taking them.
  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