Ethosuximide, Serum or Plasma

CPT: 80168
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Test Details


  • Zarontin®


Evaluate toxicity; monitor therapeutic levels


Immunoassay (IA)

Reference Interval

Therapeutic: 40−100 μg/mL

Critical Value

Potentially toxic: >100 μg/mL

Additional Information

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

Specimen Requirements


Serum or plasma


1 mL

Minimum Volume

0.6 mL


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.


Transfer separated serum or plasma to a plastic transport tube. Oral: peak: two to four hours after dose; trough: immediately prior to next dose. Peak or trough levels may be used to monitor therapy because blood levels are fairly constant.

Storage Instructions

Room temperature

Stability Requirements



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gel-barrier tube; hemolysis; lipemia; gross bacterial contamination

Clinical Information

Special Instructions

State other drugs taken by patient.


1. Bourgeois BF. Pharmacokinetic properties of current antiepileptic drugs: What improvements are needed? Neurology. 2000; 55(Suppl 3):S11-S16. 11147563


American Medical Association, Division of Drugs and Toxicology. Drug Evaluations Subscription. Chicago, Ill: AMA; Fall 1992.


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
007443 Ethosuximide (Zarontin), Serum 3616-0 007443 Ethosuximide (Zarontin), Serum ug/mL 3616-0

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