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Category Biochemistry
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 & container 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))

In renal failure, when eGFR is less than 20 mL/min/1.73m2, use
Free Phenytoin in RF = Total Phenytoin / (0.1 + (0.01 x albumin))
This laboratory reports phenytoin in mg/L and albumin in g/L. Use of other results with different units will not work with these equations.