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Blood Sciences Test


Serum or EDTA (see availability below)

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Special Instructions

Trough (pre-dose collection) is the recommended sample timing in relation to dose.

Reference Range

Quoted therapeutic ranges refer to trough levels

  • carbamazepine: 4 – 12 mg/L, half life 20 H
  • phenytoin: 10 – 20 mg/L, half life 30 H
  • All other drugs see report for therapeutic range

Test Usage

Indications for monitoring anticonvulsants vary between the different drugs.

Suggested guidelines:

  • 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.

For monitoring, measurement at steady state is preferred which usually occurs after five half-lives have elapsed.

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

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.

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.


Carbamazepine and phenytoin are measured locally

The following are referred to another laboratory for analysis:

Torbay Biochemistry:

Valproic Acid/Valproate (Serum)

Cardiff Therapeutic Drug Monitoring Service:

Phenobarbitone (Serum)

Chalfont Epilepsy Centre (available to Neurology and Paediatrics only):

Brivaracetam Level (Serum)

Oxcarbazepine Level (Serum)

Lacosamide Level (Serum)

Perampanel Level (EDTA)

Zonisamide Level (EDTA)

Topiramate Level (Serum)

Clobazam (Serum)

Ethosuximide (Serum)

Birmingham City Hospital (available to Neurology and Paediatrics only):

Lamotrigine (EDTA)

Levetiracetam (EDTA)

Turnaround Time

Carbamazepine and phenytoin: 4 days

Other tests: 5 to 10 days

Can be added on to an existing request up to 2 days following sample receipt

Specimen Labelling Procedure
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