This article is from the Epilepsy FAQ, by Andrew Patrick (email@example.com) with numerous contributions by others.
"Blood level" refers to the amount of anticonvulsant in the blood. It
is measured with a simple blood test and is used to help determine if a
patient's symptoms may be due to toxicity or to side effects of
medication. It is also used to determine if the patient is taking
enough medication to prevent seizures. The therapeutic range for
different anti-convulsants has been determined by testing blood levels
in thousands of patients whose seizures are controlled and who are not
experiencing toxic effects.