Phenytoin concentration (serum/plasma)
This test measures the amount of phenytoin, a medication used to control certain types of seizures, in your blood.
["Your doctor may check this level to ensure your seizure control is safe and effective."]
Key facts at a glance
- LOINC
- 73207-3
- Short code
- Phenytoin ind plt IgM SerPl Ql FC
- Synonyms
- Phenytoin ind plt IgM 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 This Test Is Done
Your healthcare provider may order this test if you’re taking phenytoin (brand names like Dilantin) to manage epilepsy or other seizure disorders. Regular monitoring helps prevent side effects, such as dizziness, nausea, or coordination issues, while ensuring the medication remains effective. Blood levels can also be checked if you’re experiencing unusual symptoms or if your dose needs adjustment.
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 blood. If you’re nervous, let your provider know—they can offer ways to make it more comfortable. Avoid eating or drinking for at least 8–12 hours before the test if your provider specifies it.
What to Expect During the Test
The blood sample will be taken by a healthcare professional. You might feel a slight pinch, but it usually doesn’t cause pain. The test itself takes only a few minutes. If you’re taking phenytoin, your provider may also check for other medications or substances that could affect results.
What the Results Mean
Results are compared to a general range, but exact values depend on your medication dose, health, and other factors. Too-high or too-low levels can affect how well the drug works or cause side effects. Your provider will discuss the results with you and adjust your treatment if needed.
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.
Clinical Significance
Short name
Phenytoin Level
Frequently asked questions
What causes phenytoin levels to be checked?
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