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Phenytoin concentration (serum/plasma)

Published: August 9, 2026Updated: August 17, 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 doctors monitor your dose and ensure it’s working safely to prevent seizures."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
74261-9
Short code
HEDIS 2014 Phenytoin
Synonyms
HEDIS 2014 Phenytoin
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
20
Critical range
— – 30 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 doctor may order this test if you’re taking phenytoin to manage epilepsy or other seizure disorders. It helps check if your medication is at an effective level without causing harmful side effects.

How the Test Works

A blood sample is taken from a vein in your arm. The lab uses a specialized method (likely immunoassay or chemiluminescent immunoassay) to measure phenytoin concentration in your serum or plasma.

What to Expect Before the Test

No special preparation is needed, but tell your doctor about any other medications you’re taking, as they may affect results.

Possible Results and Interpretation

Results are typically reported in micrograms per milliliter (µg/mL). Your doctor will compare these results to your specific needs and any guidelines for your condition.

Reference range

Therapeutic: 10-20 mcg/mLN/A

Specimen: serum

No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Phenytoin Level

Frequently asked questions

What does a low phenytoin level mean?
A low level may indicate your medication isn’t working as intended, and your doctor might adjust your dose or check for other issues like poor absorption or drug interactions.
What does a high phenytoin level mean?
High levels can cause side effects like confusion, rash, or trouble with coordination. Your doctor will work with you to lower the dose or switch medications if needed.
How often should I get this test?
Your doctor will determine how often you need testing based on your treatment plan, seizure control, and how your body responds to the medication. Follow their advice closely.
  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 17, 2026. · Read our editorial policy