Phenytoin
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Measures the concentration of phenytoin in the blood
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🧪 What is the Phenytoin Test?
The Phenytoin test measures the concentration of phenytoin in the blood.
Phenytoin is an anticonvulsant medication used to treat and prevent seizures, particularly tonic-clonic (grand mal) and complex partial seizures.
It has a narrow therapeutic range, meaning that too little may not control seizures, and too much can lead to toxicity.
❓ Why is the Test Done?
To:
- Monitor therapeutic drug levels
- Prevent toxicity or subtherapeutic dosing
- Adjust doses in patients with:
- New seizures
- Liver/kidney disease
- Drug interactions
- Investigate signs of phenytoin toxicity
📊 Normal (Therapeutic) Range
Serum Level | Interpretation |
|---|---|
10–20 µg/mL | Therapeutic range |
<10 µg/mL | Possibly ineffective for seizure control |
>20 µg/mL | Increased risk of toxicity |
Some labs may also measure free phenytoin (unbound), especially in patients with low albumin or renal issues.
⚠️ Signs of Phenytoin Toxicity
- >20 µg/mL: Nystagmus, ataxia
- >30 µg/mL: Slurred speech, confusion
- >40 µg/mL: Seizures, coma
🧠 Associated Systems & Conditions
System | Relevance |
|---|---|
Neurologic | Seizure control, anticonvulsant therapy |
Hepatic | Metabolized in liver; affected by liver disease |
Hematologic | Rarely may cause blood dyscrasias |
Dermatologic | Stevens-Johnson syndrome (severe reaction) |
🔄 Related / Follow-Up Tests
- Liver Function Test (LFT) – metabolism monitoring
- Albumin – affects free phenytoin level
- Renal Function Test (RFT) – affects clearance
- EEG – for seizure evaluation
- CBC – to rule out rare side effects
⏰ Timing of Sample Collection
- Ideally taken just before the next dose (trough level)
- 5–7 days after starting or adjusting dose (steady-state)
✅ Fasting Required?
Test | Fasting Required |
|---|---|
Phenytoin | ❌ No, but pre-dose (trough) preferred |
📝 Summary Table
Parameter | Details |
|---|---|
What | Measures blood level of anticonvulsant phenytoin |
Why | Monitor seizure control and prevent toxicity |
Therapeutic Range | 10–20 µg/mL |
Toxicity | >20 µg/mL (mild), >30–40 µg/mL (severe) |
Associated Systems | Neurologic, hepatic, renal |
Follow-up Tests | LFT, albumin, renal panel, EEG |
Fasting Required | ❌ No – but pre-dose timing is important |
How our test process works!
