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Phenytoin (Dilantin) Concentration

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 safe and effective."]

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

Key facts at a glance

LOINC
64718-0
Short code
Receive help alcohol drug prob PhenX
Synonyms
Receive help alcohol drug prob PhenX
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
phenx
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 a phenytoin test if you’re taking phenytoin to manage epilepsy or other seizure disorders. Regular monitoring helps prevent side effects, such as dizziness or coordination issues, and ensures the medication stays at an effective dose.

How the Test Works

A blood sample is taken from a vein in your arm. The lab uses a sensitive method (likely immunoassay or high-performance liquid chromatography) to measure phenytoin levels in your serum or plasma.

What to Expect Before the Test

No special preparation is needed, but inform your doctor if you’re taking other medications, have liver or kidney problems, or are pregnant.

Possible Side Effects of Medication

Phenytoin can cause side effects like nausea, dizziness, or skin rashes. High or low levels may worsen these effects or lead to more serious issues, such as heart rhythm problems or bone density changes.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Phenytoin Level

Frequently asked questions

What does a high or low phenytoin level mean?
High levels may indicate overdose or poor metabolism, while low levels could mean the medication isn’t working as intended. Your doctor will adjust your dose based on these results.
Can I eat or drink before the test?
You can eat and drink normally unless your doctor specifies otherwise. Avoid alcohol, as it may affect phenytoin metabolism.
How long will it take to get results?
Results are typically available within 1–3 business days, depending on your lab.
  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