Phenytoin is a primary anticonvulsant for prophylaxis and
treatment of partial and generalised tonic-clonic seizures. It is
particularly appropriate where once daily dosing is an advantage.
It does however have unpleasant comestic effects on long term
treatment. The main indications for monitoring are:
- On initiating therapy
- During I.V. therapy in status epilepticus
- Unexpected deterioration in seizure control
- As an adjunct to the diagnosis of toxicity
- When interacting drugs are added or withdrawn
- In pregnancy
Phenytoin is metabolised by the hepatic mixed-function oxidase
system, which has a limited capacity and can become saturated at
phenytoin concentrations within the target range. When levels are
close to the saturation point a small increase in dose can result
in a marked increase in serum concentration: this saturation point
varies widely between patients.
Request on ICE
Please state dosage and time of last dose on request form.
Available at all times. Please contact the laboratory regarding
A trough sample is required - i.e. pre-dose
Steady state levels. This is 4-5 days following a change in
Serum in yellow top (SST) tube
Samples should be analysed same day if left on gel.
Causes for Rejection
5 - 20 mg/L
NOTE THAT LEVELS ARE REPORTED IN mg/L; to
convert mg/L to µmol/L multiply by 3.96
This range is indicative only, and some patients tolerate higher
levels and require them to achieve effective control. Other
patients are adequately controlled at lower concentrations and
there is no need to use higher doses in such patients.
Phenytoin is over 90% protein-bound in healthy adults on
monotherapy, but binding is substantially reduced in neonates,
pregnancy, renal or hepatic disease. In such cases, effective or
toxic free drug levels are obtained at lower total serum phenytoin
Symptoms of neurotoxicity (nausea, vomiting, tremor. ataxia) are
increasingly frequent as levels exceed 20 mg/L
Following overdose and confirmation of toxicity there is not a
case for continuous monitoring of levels to decide when to resume
therapy. Phenytoin half-life is usually between 24 and 48 hours but
due to the saturation kinetics displayed, levels will gradually
fall and then plummet i.e. half-life will be dependant on serum
levels. Usually, phenytoin is stopped for 2 to 3 days and then the
normal dose resumed. Serum levels should be monitored 7-10 days
later after steady state has been obtained.