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Phenytoin (Serum) Concentration

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

This test measures the amount of phenytoin, an antiseizure medication, in your blood. It helps doctors monitor your dose to prevent seizures or toxicity.

Quick summary

["This test ensures your antiseizure medication levels stay safe and effective."]

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

Key facts at a glance

LOINC
14887-4
Short code
Primidone SerPl-sCnc
Synonyms
Primidone 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 Matters

Phenytoin is used to control certain types of seizures. Too little may not work well, while too much can cause serious side effects like dizziness, confusion, or heart rhythm issues. This test helps balance your dose for the best protection against seizures without unnecessary risks.

How It’s Done

A blood sample is taken from a vein in your arm. The lab then measures the phenytoin concentration in your serum (the liquid part of blood after blood cells and clots are removed).

What to Expect Before the Test

There are no special preparations needed. You may have a small bruise or sore spot where the needle went in, but this usually fades quickly. If you’re on phenytoin, your doctor may adjust your dose based on this test result.

What the Results Mean

Results are typically reported in micrograms per milliliter (µg/mL). Your doctor will interpret the result in relation to your medical condition and other medications. Common target ranges vary by patient and condition, so your doctor will guide you.

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 doctor will determine how often you need this test based on your seizure control, other medications, and how well you’re tolerating phenytoin. Regular monitoring helps prevent both underdosing (seizures) and overdosing (side effects).
What if my results are too high or too low?
If your levels are too high, your doctor may reduce your dose or switch to a different medication. If levels are too low, they may increase your dose under close supervision. Never change your medication without consulting your doctor.
Does this test interact with other medications?
Yes, phenytoin can interact with many other medications, including birth control pills, antibiotics, and heart medications. Always tell your doctor about all medications you’re taking, including over-the-counter drugs and supplements, as they may affect your phenytoin levels.
  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