Ethosuximide, Serum or Plasma

Serum or plasma

1 mL

0.6 mL (Note: This volume does not allow for repeat testing.)

Red-top tube, lavender-top (EDTA) tube, or green-top (heparin) tube. Do not use a gel-barrier tube. The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.

Evaluate toxicity; monitor therapeutic levels

Immunoassay (IA)

Therapeutic: 40−100 μg/mL

Ethosuximide is well absorbed orally, and the peak plasma concentration occurs in one to four hours. It is minimally bound to plasma protein and eliminated primarily via hepatic metabolism with 10% to 20% of the administered dose excreted unchanged in the urine. The half-life is variable but averages 52 to 56 hours in adults and 32 to 41 hours in children. Breast milk concentrations are slightly less than corresponding plasma concentrations, but problems related to breast-feeding in humans have not been documented. Control of absence seizures usually is achieved with plasma concentrations of 40−100 μg/mL, but concentrations up to 160 μg/mL may be required and are tolerated in some patients. As is the case with other antiepileptic drugs metabolized by the P450 enzyme system, co-administration of carbamazepine, phenytoin, phenobarbital, or primidone will result in decreased ethosuximide levels, whereas valproic acid can increase ethosuximide levels.¹