Serum or EDTA (see availability below)
Trough (pre-dose collection) is the recommended sample timing in relation to dose.
Quoted therapeutic ranges refer to trough levels
Indications for monitoring anticonvulsants vary between the different drugs.
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.
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:
Valproic Acid/Valproate (Serum)
Cardiff Therapeutic Drug Monitoring Service:
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)
Birmingham City Hospital (available to Neurology and Paediatrics only):
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 receiptSpecimen Labelling Procedure