Phenytoin (Dilantin) Blood Test

The Phenytoin test measures the level of phenytoin (Dilantin) in the blood. Phenytoin is an anticonvulsant (antiepileptic) medication used to control seizures in conditions like epilepsy. Monitoring phenytoin levels is crucial because too little can lead to seizures, while too much can cause toxicity.

Why is the Test Done?

✅ Ensure therapeutic drug levels – Prevent seizures while avoiding toxicity
✅ Monitor treatment effectiveness – Especially if seizure control is poor
✅ Check for toxicity symptoms – Drowsiness, confusion, unsteady movement
✅ Assess drug interactions – Other medications can affect phenytoin levels
✅ Monitor liver or kidney function – These organs process phenytoin

Therapeutic Range & Interpretation

📌 Free Phenytoin (Unbound) Levels: For patients with liver disease, low albumin, or kidney issues, a free phenytoin test may be needed. The normal range for free phenytoin is 1-2 mcg/mL.

Causes of Abnormal Phenytoin Levels

🔺 High Phenytoin Levels (>20 mcg/mL) – Toxicity Risk

🔹 Overdose or excessive dosing
🔹 Liver disease – Slower drug metabolism
🔹 Kidney disease – Reduced drug clearance
🔹 Drug interactions – Some antibiotics, antifungals, and antidepressants increase phenytoin levels
🔹 Dehydration – Can cause temporary elevation

📌 Symptoms of Phenytoin Toxicity:
⚠ Nausea, dizziness, blurred vision, slurred speech
⚠ Unsteady walking (ataxia), confusion
⚠ Severe cases: Seizures, coma

🔻 Low Phenytoin Levels (<10 mcg/mL) – Risk of Seizures

🔹 Missed doses or poor medication adherence
🔹 Rapid metabolism (genetic factors)
🔹 Drug interactions (e.g., carbamazepine, rifampin) – Can lower phenytoin levels
🔹 Malabsorption or poor digestion – Certain gastrointestinal conditions

📌 Low levels increase seizure risk, especially in epilepsy patients.

Next Steps Based on Results

🔬 If levels are too high:
✔ Reduce dosage (under medical guidance)
✔ Check for drug interactions
✔ Monitor liver & kidney function

💊 If levels are too low:
✔ Adjust dosage
✔ Assess for drug interactions that reduce phenytoin levels
✔ Ensure proper medication adherence