Phenytoin concentration in serum/plasma (quantitative)
This test measures the amount of phenytoin, a medication used to control certain types of seizures, in your blood.
["Phenytoin levels help ensure your medication dose is properly managed for seizure control."]
Key facts at a glance
- LOINC
- 60482-7
- Short code
- HEDIS 2011-12 MPM-E Phenytoin
- Synonyms
- HEDIS 2011-12 MPM-E 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 healthcare provider may order this test if you’re taking phenytoin (brand names include Dilantin®) to check if your blood levels are within the safe range. Proper dosing is important to prevent seizures while minimizing side effects. This test may also be done if you’re experiencing unusual symptoms like dizziness, confusion, or rash that could be linked to your medication.
What to Expect Before the Test
No special preparation is needed. You may have a small needle inserted into a vein in your arm or hand for blood collection. If a fingerstick is used, a small amount of blood is taken from your fingertip. The process is quick and usually painless.
What the Test Results Mean
Results help your doctor determine if your phenytoin levels are too high (which can cause toxicity) or too low (which may mean your seizures aren’t well-controlled). Your doctor will discuss the results with you and adjust your medication if needed.
Possible Side Effects or Risks
While rare, very high levels of phenytoin can cause drowsiness, blurred vision, or coordination problems. Low levels may not provide enough seizure control. Always follow your doctor’s instructions for taking this medication.
Reference range
Therapeutic: 10-20 mcg/mLN/A
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.
Maintaining appropriate phenytoin levels is critical for managing epilepsy or seizure disorders. Too little medication may lead to uncontrolled seizures, while too much can cause toxicity, affecting the nervous system, liver, or bones. Regular monitoring helps balance safety and effectiveness.
Short name
Phenytoin Level
Frequently asked questions
What does a ‘normal’ result mean?
How often should I get this test?
Can I eat or drink before this test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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