Interpretive Handbook

Test 8769 :
Ethosuximide, Serum

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Ethosuximide (Zarontin) is used in the treatment of absence (petit mal) seizures, although valproic acid and methsuximide are used more frequently for this condition. Ethosuximide is completely absorbed from the gastrointestinal tract, reaching a peak plasma concentration in 1 to 7 hours.  


Approximately 10% to 20% of the drug is excreted unchanged in the urine; the remainder is metabolized by hepatic microsomal enzymes. The volume of distribution of ethosuximide is 0.7 L/kg, and its half-life is 40 to 50 hours. Little ethosuximide circulating in the blood is bound to protein.


Ethosuximide produces a barbiturate-like toxicity, characterized by central nervous system and respiratory depression, nausea, and vomiting when the blood level is > or =101 mcg/mL.

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring therapy


Determining compliance


Assessing toxicity

Interpretation Provides information to assist in interpretation of the test results

Dosage is guided by blood levels; the therapeutic range for ethosuximide is 40 to 100 mcg/mL


Toxic concentration: > or =101 mcg/mL

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

No significant cautionary statements

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Therapeutic concentration: 40-100 mcg/mL

Toxic concentration: > or =101 mcg/mL

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Patsalos PN, Berry DJ, Bourgeois BF, et al: Antiepileptic drugs-best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug  monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia 2008;49(7):1239-1276

2. Moyer TP: Therapeutic drug monitoring. In Tietz Textbook of Clinical Chemistry. Third edition. Edited by CA Burtis, ER Ashwood. WB Saunders Company, Philadelphia, 1999, pp 862-905