Test Background Phenytoin is indicated in all forms of epilepsy except absence seizures. It is licensed for status epilepticus and trigeminal neuralgia therapy when carbamazepine is inappropriate. Phenytoin is the drug of choice to treat and prevent tonic-clonic and psychomotor seizures. If phenytoin alone will not prevent seizure activity, co-administration with phenobarbital is usually effective.
Clinical Indications Therapeutic drug monitoring Assessment of compliance and toxicity (symptoms of overdosage include: nystagmus, ataxia and dysarthria, progressing to tremor, hyper-reflexia and lethargy)
Reference Range 5-20 mg/L
Sample Required SST (gold top)
Sample Volume 0.5 mL
Turnaround Time 1 day
Total phenytoin concentration is measured and reported. In patients with hypoalbuminaemia and / or severe renal impairment, the total phenytoin result may be misleading and needs to be adjusted. This can be done using the Sheiner-Tozer equations below.
Free Phenytoin = Total Phenytoin / (0.1 + (0.02 x albumin))