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Diphenylhydantoin (Phenytoin) Serum 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. It helps doctors monitor your dose to keep it safe and effective.

Quick summary

["Phenytoin levels help ensure your seizure medication is at the right amount for your body."]

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

Key facts at a glance

LOINC
73414-5
Short code
Diphenhy ind plt IgM SerPl Ql FC
Synonyms
Diphenhy ind plt IgM SerPl Ql FC
Category
hematology
Sample
serum
Container
Red-top tube (SST)
Units
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 This Test Is Done

Your doctor may order this test if you’re taking phenytoin (brand names include Dilantin®) to manage epilepsy or other seizure disorders. Regular monitoring helps prevent side effects like dizziness, rash, or irregular heart rhythms while ensuring the medication works properly.

How the Test Works

A blood sample is taken from a vein in your arm. The lab uses a blood test to measure phenytoin levels in your serum. The result helps your healthcare provider adjust your dose if needed.

What to Expect Before the Test

No special preparation is usually needed, but tell your doctor about all medications, supplements, or recent illnesses—especially liver or kidney problems, as they may affect results or your medication.

Possible Results and Interpretation

Results are typically reported in micrograms per milliliter (mcg/mL). Too-high or too-low levels can affect how well the medication works or cause side effects. Your doctor will discuss your specific results with you.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Phenytoin levels are critical for managing epilepsy safely. Too little may mean seizures aren’t controlled, while too much can cause toxicity, including nausea, ataxia (balance issues), or severe allergic reactions. Regular monitoring helps balance effectiveness and safety.

Short name

Phenytoin Level

Frequently asked questions

How often should I get this test?
Your doctor will determine the frequency based on your medication regimen, health status, and how well you’re responding to the treatment. Typically, tests may be done weekly or monthly, depending on your situation.
What happens if my phenytoin level is too high or too low?
If levels are too high, your doctor may reduce your dose or switch medications. If levels are too low, they may increase your dose or check for other issues, like poor absorption or drug interactions.
Can I drink alcohol before this test?
It’s best to avoid alcohol before the test, as it can affect blood test results and may interact with phenytoin. Always follow your doctor’s instructions.
  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