Phenytoin (Dilantin) concentration by turbidimetric assay
This test measures the amount of phenytoin, a medication used to control certain types of seizures, in your blood. It helps doctors monitor your drug levels to ensure they’re within the safe range for your treatment.
["Phenytoin levels help prevent dangerous side effects from too much or too little medication."]
Key facts at a glance
- LOINC
- 108365-8
- Short code
- Fenetylline Msmt Ur
- Synonyms
- Fenetylline Msmt Ur
- Category
- chemistry
- Sample
- urine
- Container
- Sterile urine container
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Specimen volume
- 10 mL
- Stability
- Room temperature: 2h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 0
- Lipemia max
- 0
- Icterus max
- 0
- TAT
- 4–24 hours
- Order
- routine
Why This Test Is Done
Your doctor may order this test if you’re taking phenytoin to manage epilepsy or other seizure disorders. Regular monitoring helps adjust your dose to keep seizures under control while minimizing side effects. It’s often done when you start taking the medication, after changes in your dose, or if you experience unusual symptoms like dizziness, confusion, or slow heart rate.
What to Expect Before the Test
No special preparation is needed. You may have a small needle inserted into a vein (usually in your arm) to draw blood. If you’re concerned about needles, let your healthcare provider know. The test itself takes only a few minutes.
What the Test Results Mean
Results are compared to a general reference range, but your specific target range depends on your medical needs. Too much phenytoin can cause toxicity, while too little may not control seizures effectively. Your doctor will interpret the results in relation to your symptoms and treatment plan.
Common Side Effects to Watch For
Phenytoin can cause side effects like drowsiness, nausea, or gum overgrowth. If you experience severe symptoms such as rash, rapid heartbeat, or trouble breathing, seek medical attention immediately.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Maintaining proper phenytoin levels is critical for seizure control and reducing risks of overdose or underdose. Blood levels help doctors balance efficacy and safety, especially in patients with kidney or liver issues, who may need dose adjustments.
Short name
Phenytoin Level
Frequently asked questions
How often should I get this test?
What happens if my levels are 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 16, 2026. · Read our editorial policy