Sign in →

Test Code LAB1230480 Ethosuximide, Serum

Additional Codes

Mayo Code: ETHSX

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Useful For

Monitoring therapy


Determining compliance


Assessing toxicity

Reporting Name

Ethosuximide, S

Specimen Type


Specimen Required


Preferred: Serum gel

Acceptable: Red top

Specimen Volume: 0.5 mL

Collection Instructions:

1. Serum gel tubes should be centrifuged within 2 hours of collection.

2. Red-top tubes should be centrifuged and aliquoted within 2 hours of collection.

Specimen Minimum Volume

0.25 mL

Specimen Stability Information

Specimen Type Temperature Time
Serum Refrigerated (preferred) 14 days
  Frozen  28 days
  Ambient  14 days

Reject Due To


Mild OK; Gross reject







Clinical Information

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 >150 mcg/mL.

Reference Values

Therapeutic: 40-100 mcg/mL

Critical value: >150 mcg/mL


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


Toxic concentration: >150 mcg/mL.

Clinical Reference

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

Method Description

The assay is a homogeneous enzyme immunoassay technique used for the analysis of specific compounds in biological fluids. The assay is based on competition between drug in the sample and drug labeled with the enzyme glucose-6-phosphate dehydrogenase (G6PDH) for antibody binding sites. Enzyme activity decreases upon binding to the antibody, so the drug concentration in the sample can be measured in terms of enzyme activity. Active enzyme converts oxidized nicotinamide adenine dinucleotide (NAD) to NADH (the reduced form of NAD), resulting in an absorbance change that is measured spectrophotometrically. Endogenous serum G6PDH does not interfere, because the coenzyme functions only with the bacterial (Leuconostoc mesenteroides) enzyme employed in the assay.(Package insert: Siemens Ethosuximide reagent, Siemens Healthcare Diagnostics Inc., Newark, DE)

Method Name

Enzyme-Multiplied Immunoassay Technique (EMIT)

Day(s) and Time(s) Performed

Monday through Sunday; Continuously

Analytic Time

Same day/1 day

Test Classification

This test has been cleared or approved by the U.S. Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

LOINC Code Information

Test ID Test Order Name Order LOINC Value
ETHSX Ethosuximide, S 3616-0


Result ID Test Result Name Result LOINC Value
ETHSX Ethosuximide, S 3616-0

CPT Code Information



If not ordering electronically, complete, print, and send a Neurology Specialty Testing Client Test Request (T732) with the specimen.

Laboratory Test Directory Additional Information:

Added 4/16/2018 - Replaces In-House Test LAB683 (683, 4019)