Specimen must be collected pre-dose or at least 10- 15 hours after the last dose of clozapine.
While there is little evidence that doses need to be individualised based on plasma drug concentrations plasma levels may be useful in the case of possible drug interaction, overdose or lack of response with usual doses without evidence of toxicity.
The incidence of CNS side effects and seizures increases at concentrations over 2000 nmol/L and increases further over 3000 nmol/L.
Drugs that decrease clozapine concentration incude phenytoin, carbamazepine and valproate and probably include rifampicin, phenobarbitone and tobacco smoke.
Drugs that increase clozapine concentration include cimetidine, fluvoxamine, erythromycin anf fluoxetine and probably include azole antifungal agents and diltiazem.
