Indications for monitoring/checking drug levels of antiepileptics

NICE (1,2) state that:

• in adults - regular blood test monitoring in adults is not recommended as routine, and should be done only if clinically indicated
• in children - regular blood test monitoring in children is not recommended as routine, and should be done only if clinically indicated and recommended by the specialist
• indications for drug levels:
  • detection of non-adherence to the prescribed medication
  • suspected toxicity
  • adjustment of phenytoin dose
  • management of pharmacokinetic interactions
  • specific clinical conditions, for example, status epilepticus, organ failure and pregnancy
• examples of blood tests include:
  • before surgery - clotting studies in those on valproate
  • full blood count, electrolytes, liver enzymes, vitamin D levels, and other tests of bone metabolism (for example, serum calcium and alkaline phosphatase) every 2-5 years for adults taking enzyme-inducing drugs
• asymptomatic minor abnormalities in test results are not necessarily an indication for changes in medication

Note that the timing of drug level sampling is critical, and should always be indicated on the request form. Particularly important are:

• whether a trough or peak is being measured
• bloods taken at the time of symptoms

Drug levels are interpreted in the light of therapeutic ranges; however it must be remembered that ranges are a statistical statement and the context of the individual must be bourne in mind.

Reference:

1. NICE (October 2004).The epilepsies: the diagnosis and management of the epilepsies in adults and children in primary and secondary care.
2. NICE (April 2018). Epilepsies: diagnosis and management