Phenytoin concentration (serum/plasma)
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 appropriate for managing seizures safely."]
Key facts at a glance
- LOINC
- 54057-5
- Short code
- HEDIS 2009-10 MPM-E Phenytoin
- Synonyms
- HEDIS 2009-10 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 Is This Test Done?
Your healthcare provider may order this test to check if your phenytoin levels are within the safe range. Too little phenytoin can reduce seizure control, while too much may cause side effects like dizziness, nausea, or coordination problems. Regular monitoring helps balance effectiveness and safety.
What to Expect Before the Test
No special preparation is needed. You may have a small needle inserted into a vein (venipuncture) in your arm or hand. Blood is drawn into a tube. If you’re concerned about needles, let your healthcare provider know.
What to Expect During the Test
The blood sample is sent to a lab for analysis using an immunoassay method to detect phenytoin. Results are typically available within 24–48 hours, depending on lab processes.
What the Results Mean
Results are compared to a general reference range, but exact values can vary by lab. Your healthcare provider will interpret the results in the context of your medical history, symptoms, and other medications.
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.
Phenytoin is a potent medication that requires careful dosing to avoid toxicity. Low levels may indicate insufficient seizure control, while elevated levels can cause toxicity, including rashes, ataxia (balance issues), or cardiac problems. Regular monitoring helps prevent these risks and optimize treatment.
Short name
Phenytoin Level
Frequently asked questions
How often should I get this test?
What happens if my phenytoin level is too high or too low?
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