Description
Why It’s Done
Your doctor may order this test to:
• Monitor drug levels in people taking phenytoin long-term.
• Adjust dosage to achieve optimal seizure control.
• Check for toxicity if you have symptoms like dizziness, confusion, slurred speech, or unsteady movements.
• Confirm compliance with medication in patients with uncontrolled seizures.
Therapeutic range for phenytoin:
Total phenytoin: 10–20 mcg/mL
Free phenytoin: 1–2 mcg/mL (measured in special cases)
Preparation
• Timing: A trough level (just before the next dose) is usually recommended for accurate monitoring.
• Inform your doctor about:
o Other medications (as many drugs can interact with phenytoin).
o Recent dosage changes or missed doses.
• No fasting is generally required unless advised otherwise.
• A blood sample is drawn from a vein in your arm.
Always take the medication as prescribed and inform the lab or physician about the exact timing of your last dose.
