What this test measures
Phenytoin (marketed in India as Eptoin and Dilantin) is one of the oldest and most widely used anticonvulsants for focal and generalised tonic-clonic seizures, and intravenously for status epilepticus. The blood level is reported as total phenytoin (typically 10–20 mg/L therapeutic) or free phenytoin (1–2 mg/L therapeutic; ~10% of total).
Phenytoin has narrow therapeutic range and zero-order saturation kinetics — small dose increases at higher levels can cause disproportionate level rises and toxicity. Free phenytoin is preferred in patients with low albumin, renal failure, pregnancy, or concomitant valproate use because protein-binding is altered. Therapeutic drug monitoring (TDM) is recommended in all these situations and whenever toxicity, breakthrough seizures, or drug interactions are suspected.
Why it matters
Phenytoin requires regular monitoring because: (1) Narrow therapeutic window — toxicity (nystagmus, ataxia, confusion, coma) occurs at levels >20–25 mg/L; (2) Saturation kinetics — going from 18 to 22 mg/L may require only a small dose change but precipitate toxicity; (3) Many drug interactions — phenytoin induces CYP3A4 and reduces levels of warfarin (initially raises INR, then can drop it), oral contraceptives, antiretrovirals, ciclosporin, and many others; conversely, valproate, isoniazid, and fluconazole raise phenytoin levels; (4) Pregnancy — increased clearance often requires dose escalation, but lower albumin requires free-level monitoring.
For Indian patients on long-term phenytoin (often started for childhood-onset epilepsy or post-traumatic seizures), annual TDM plus any time symptoms change (drowsiness, ataxia, breakthrough seizure) is standard of care. Anyone on phenytoin who is also on warfarin needs especially careful coagulation monitoring.
How to prepare
Take the sample as a "trough level" — just before the next dose (typically morning, before the morning dose). Do not skip the dose; just time the sample correctly. Disclose all current medications, recent illness, pregnancy, and any symptoms (drowsiness, double vision, unsteady gait, confusion).
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Phenytoin (Total) (mg/L (or µg/mL))[1][2] | Therapeutic: 10–20 mg/L | Sub-therapeutic level — risk of breakthrough seizures. Causes: non-adherence (most common), drug interactions (CYP inducers — rifampicin, carbamazepine), pregnancy (increased clearance), enteral feeding (impairs absorption), low albumin (more free drug but total may appear low). | Supra-therapeutic: 20–30 mg/L (nystagmus, mild ataxia); 30–40 (confusion, sedation, severe ataxia); >40 (stupor, coma, seizures from toxicity). Causes: dose increases, hepatic impairment, drug interactions (valproate, isoniazid, fluconazole, amiodarone), genetic polymorphisms in CYP2C9/CYP2C19. |
| Phenytoin (Free) (mg/L) | Therapeutic: 1–2 mg/L | Sub-therapeutic free level — adjust dose; check adherence. | High free level despite normal total — low albumin (cirrhosis, nephrotic syndrome), valproate co-administration, uraemia. Use free level for clinical decisions in these settings. |
Phenytoin level interpretation and toxicity
| Total level (mg/L) | Clinical effect |
|---|---|
| < 10 | Sub-therapeutic — breakthrough seizure risk |
| 10 – 20 | Therapeutic — target range |
| 20 – 30 | Nystagmus, mild ataxia |
| 30 – 40 | Confusion, sedation, severe ataxia |
| > 40 | Stupor, coma, paradoxical seizures from toxicity |
Frequently asked questions
When should I get a phenytoin level?
Routinely once at steady-state after starting or changing dose (5–10 days). Then annually if stable. Also any time you have breakthrough seizures, new symptoms (drowsiness, ataxia, double vision), pregnancy, illness, or new medications.
When should I time the blood sample?
Just before your next dose — a "trough level". Most labs and patients schedule it in the morning before the morning dose. Random levels are still informative but trough is best.
Do I need to fast?
No, but disclose meal timing if relevant — phenytoin oral suspension is affected by enteral feeds.
My phenytoin level is 25 mg/L and I am feeling fine. Should I reduce the dose?
25 mg/L is just above therapeutic range and may produce subtle nystagmus. If you are completely asymptomatic, your doctor may keep the dose; if there is any nystagmus, ataxia, or sleepiness, dose reduction is appropriate. Do not change the dose without your doctor.
Can phenytoin affect warfarin?
Yes — significantly. Phenytoin initially can displace warfarin from albumin and increase INR (bleeding risk) and then over weeks induces CYP enzymes and reduces warfarin effect (clotting risk). Anyone on both needs frequent INR monitoring.
What other drugs interact with phenytoin?
Many. Drugs that raise phenytoin: valproate, isoniazid, fluconazole, amiodarone, omeprazole. Drugs that lower phenytoin: rifampicin, carbamazepine, ritonavir, enteral feeds. Phenytoin lowers: warfarin (long-term), oral contraceptives, ciclosporin, antiretrovirals.
Why is free phenytoin measured in pregnancy?
Pregnancy lowers serum albumin and increases the free (active) drug fraction. Total levels can mislead — the free level is more meaningful clinically.
How long does the report take?
Most NABL labs report phenytoin level within 24 hours.
Related Coagulation tests
Tests commonly ordered alongside PHENYTOIN / DILANTIN / EPTOIN, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Phenytoin · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Phenytoin Level · accessed 2026-05-30T00:00:00.000Z
- American Epilepsy Society — Therapeutic Drug Monitoring · accessed 2026-05-30T00:00:00.000Z
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