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Anticonvulsants

Blood Sciences Test


Specimen

Serum

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Special Instructions

Specimens must be collected at least 8 hours after last dose, preferably just before next dose e.g. delay morning dose until after morning blood collect.

Reference Range

Quoted therapeutic ranges refer to trough levels

  • carbamazepine (Tegretol): 4 – 12 mg/L, half life 20 H
  • ethososuximide (Zarontin): 280-700 ug/L, half life 30 H
  • phenobarbitone: 10 – 40 mg/L, half life 100 H
  • phenytoin (Dilantin): 5 – 20 mg/L, half life 30 H
  • primidone (Mysoline): measured as phenobarbitone
  • sodium valproate (Epilim): See note below

Test Usage

Sodium Valproate

There is no relationship between serum concentration and therapeutic response.

The only indications for measuring sodium valproate are to check compliance or to investigate toxicity.

General:
Indications for monitoring anticonvulsants vary between the different drugs.

Suggested guide-lines:

  • monitoring initial stabilisation or change of dose: phenytoin, carbamazepine, phenobarbitone
  • suspected toxicity: all drugs
  • suspected non-compliance: all drugs
  • failure to control seizures: all drugs
  • ongoing routine monitoring: phenytoin only, and even this may not be essential.

The therapeutic range is a guide only. Levels below the range may control seizures; levels above the range may not be toxic and may be necessary for control.

When changing doses, retesting should be delayed a week or so till a new equilibrium develops.

Phenytoin, carbamazepine and phenobarbitone are potent inducers of hepatic enzymes thereby raising serum levels of GGT and ALP.

The raised enzymes are regarded as an acceptable side-effect.

Primidone (qv) is measured as phenobarbitone which is the active metabolite.

Fosphenytoin cross reacts with the phenytoin assay, it is recommended that samples for serum phenytoin measurements be collected at least 2 hours after an intravenous dose of fosphenytoin and at least 4 hours after an intramuscular dose.

Availability

Local / (Valporate Referred test)

Turnaround Time

4 days

Can be added on to an existing request up to 5 days following sample receipt

Specimen Labelling Procedure
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