Synonyms |
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Epanutin
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Clinical
Indications |
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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:
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On
initiating therapy
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During
I.V. therapy in status epilepticus
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Unexpected
deterioration in seizure control
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As
an adjunct to the diagnosis of toxicity
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When
interacting drugs are added or withdrawn
-
In
pregnancy
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Drug Kinetics |
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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.
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Request
Form |
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Combined Pathology Blood form
(Yellow/Black or Blue for GP's).
Please state dosage on request form.
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Availability |
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On
request during routine hours. Laboratory must be contacted
regarding urgent requests.
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Specific
Criteria |
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None
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Patient
Preparation |
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Steady
state levels. This is 4-5 days following a change in dose.
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Turnaround
Time |
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Same Day (Monday to Friday).
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Specimen |
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Serum
|
Volume |
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2 ml
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Container |
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Yellow top (SST) tube
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Lab.
Handling |
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Samples
should be analysed same day if left on gel.
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Causes
for Rejection |
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Unlabelled sample
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Target
Range |
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10 -
20 mg/L. 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
pheytoin levels.
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Toxicity |
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Symptoms
of neurotoxicity (nausea, vomiting, tremor. ataxia) are
increasingly frequent as levels exceed 20 mg/L
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Unit
Conversion |
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To convert from
umol/L to mg/L
multiply by 0.253
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Overdose |
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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.
An information
sheet is available from the National Poisons Service.
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