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Test Code ETHOS / ETX - M Ethosuximide, Serum

Specimen Required

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

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 the serum aliquoted into a plastic vial within 2 hours of collection.

Secondary ID


Useful For

Monitoring ethosuximide therapy


Determining compliance


Assessing ethosuximide toxicity

Method Name

Enzyme-Multiplied Immunoassay Technique (EMIT)

Reporting Name

Ethosuximide, S

Specimen Type


Specimen Minimum Volume

0.25 mL

Specimen Stability Information

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

Reject Due To

Gross hemolysis 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 greater than 150 mcg/mL.

Reference Values

Therapeutic: 40-100 mcg/mL

Critical value: >150 mcg/mL


No significant cautionary statements

Day(s) and Time(s) Performed

Monday through Saturday

Analytic Time

Same day/1 day
NorDx Test Catalog Note:

Analytic time for send out tests is the time it will take to perform testing once it has arrived at the performing reference lab. Please add 1 to 2 days from time of collection to allow for receipt at NorDx central lab and shipment of specimen, add another day if specimen is collected on the day before a weekend or holiday

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information


LOINC Code Information

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


Result ID Test Result Name Result LOINC Value
8769 Ethosuximide, S 3616-0


If not ordering electronically, complete, print, and send a Therapeutics Test Request (T831) with the specimen.