Phenytoin (Epanutin)

Clinical Chemistry

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Description

Phenytoin is used to control seizures. There is large inter- and intra-individual variation in relationship between phenytoin dose and its plasma concentration.  Where dose changes are being considered, the patient should ideally be at steady-state (up to a minimum of 21 days after initiation of therapy or last dose change) prior to sample collection.

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Indications

This guidance concerns phenytoin use to treat epilepsy. Guidance regarding use and monitoring in pregnancy is outside the scope of this guidance.

Use of phenytoin to treat other conditions is outside the scope of this guidance.

Measurement of phenytoin may be useful in the following circumstances:
- dose optimisation.
- uncontrolled seizures or suspected toxicity.
- in patient populations with greater metabolic variability/polypharmacy (children, pregnancy, elderly, severe intercurrent illness including hepatic impairment). Multiple drug interactions are known.
- where the formulation of the drug is altered.
- where there is the possibility of drug interactions.

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Sample Type

Serum, SST/Gel, minimum 2 mL ( 1 mL separated serum ), ideally pre-dose.

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Reference Range

Reference ranges are provided on the report. Alternatively, please contact the laboratory for current ranges.

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Turnaround Time

Within 1 day

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Testing Frequency

Daily

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External Notes

Ideally sample pre-dose for therapeutic drug monitoring (TDM) purposes. In cases of suspected toxicity, please indicate time of last dose and sample collection.

Note that phenytoin can be absorbed onto the gel used in 'serum separator' tubes (yellow top 'SSTs'): samples must reach the laboratory promptly after collection. Free phenytoin levels may be increased in hypoalbuminaemia.

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Patient Preparation

For TDM purposes, take samples pre-dose (ideally). Patients should have attained steady-state (see below) prior to sample collection.

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Please note: the above information is subject to change and we endeavour to keep this website up to date wherever necessary.