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Phenytoin (Antiepileptic Drug Level) in 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."]

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

Key facts at a glance

LOINC
73206-5
Short code
Phenytoin ind plt IgG SerPl Ql FC
Synonyms
Phenytoin ind plt IgG SerPl Ql FC
Category
hematology
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
Method
flow-cytometry
CPT
82784
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 Is This Test Done?

Your healthcare provider may order this test if you’re taking phenytoin to manage epilepsy or other seizure disorders. It helps check if your medication levels are within the safe range to prevent side effects or seizures.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into your arm or vein to draw a blood sample. It usually takes a few minutes.

What the Results Mean

Results help determine if your phenytoin dose is too high or too low. Your doctor will discuss these results with you and adjust your medication if needed.

Common Reasons for Testing

This test is often done regularly if you take phenytoin long-term to avoid fluctuations in your medication levels that could affect seizure control or cause toxicity.

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.

Phenytoin levels are critical because they can affect how well the drug works. Too much can cause dizziness, confusion, or other side effects, while too little may not control seizures effectively. Your doctor will guide you on maintaining safe levels.

Short name

Phenytoin Level

Frequently asked questions

Why might my phenytoin level be checked more often?
If you have certain health conditions (like liver or kidney problems), take other medications, or experience changes in how you feel while on phenytoin, your doctor may order more frequent tests to adjust your dose safely.
What happens if my phenytoin level is too high?
If your level is too high, your doctor may reduce your dose or switch medications to avoid side effects like nausea, rash, or coordination problems.
Could diet or other medications affect my phenytoin test results?
Yes, certain foods, supplements, or other drugs (like antacids or antibiotics) can change how phenytoin works in your body. Always tell your doctor about everything you take.
  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