Carbamazepine, Serum or Plasma

CPT: 80156
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Test Details

Synonyms

  • Carbatrol®
  • Epitol®
  • Equetro®
  • Tegretol®

Use

This heterocyclic (iminostilbene) compound has potent antiepileptic properties and is effective alone or with other antiepileptic drugs in partial seizures, especially complex partial seizures, generalized tonic-clonic seizures, and combinations of these seizure types. Carbamazepine generally is ineffective for absence, myoclonic, and atonic seizures. In children with symptomatic generalized epilepsy and continuous spike-and-wave discharge, these seizure types may develop or tonic-clonic seizures may increase in frequency with use of carbamazepine. It has also been noted1 that the dose of carbamazepine cannot be used as a reliable index for predicting the serum concentration of either total or free carbamazepine serum concentrations in children with epilepsy.

Comparative clinical trial data indicate that patients with partial seizures may tolerate carbamazepine better than phenobarbital and primidone, but individual responses vary. Many clinicians consider carbamazepine a drug of choice for initial therapy in idiopathic and symptomatic localization-related epilepsies, especially in children and women. This drug is increasingly preferred to phenobarbital in pediatric patients because it has less effect on cognition and behavior. It is reported to have psychotropic activity that may increase alertness and elevate mood in depressed epileptic patients, but not in otherwise normal patients. Mental improvements may be due to substitution of carbamazepine for sedative drugs, control of seizures, or a direct psychotropic effect. Carbamazepine can induce the hepatic enzymes CYP3A4, CYP1A2, and CYP2C9, resulting in decreased serum levels of many drugs (eg, alprazolam, clozapine, diazepam, haloperidol, risperidone, and tricyclic antidepressants).2

Methodology

Immunoassay

Reference Interval

Therapeutic: In conjunction with other antiepileptic drugs: 4.0−8.0 μg/mL; carbamazepine alone: 6.0−12.0 μg/mL

Critical Value

Potentially toxic: >12.0 μg/mL

Additional Information

Leukopenia may be dose related, and necessitates stopping the drug if the absolute neutrophil count falls to <1000/mm3.3 Hyponatremia may occur, especially in older patients. Patients in the first month of pregnancy are at increased risk of neural tube defects. Carbamazepine may interfere with the actions of theophylline, oral contraceptives, oral anticoagulants, or doxycycline. Conversely, there have been reports indicating fluoxetine may mediate an increase in carbamazepine plasma concentrations due to inhibition of CYP3A4.2

Specimen Requirements

Specimen

Serum or plasma

Volume

1 mL

Minimum Volume

0.3 mL

Container

Red-top 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.

Collection

Transfer separated serum or plasma to a plastic transport tube. Draw trough levels immediately prior to next dose. Due to variability of absorption, the trough level may not represent the lowest drug level during the dosing interval, so multiple determinations may be necessary in some patients.

Storage Instructions

Room temperature

Stability Requirements

Temperature

Period

Room temperature

14 days

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gel-barrier tube; hemolysis; gross lipemia; icteric specimen

Clinical Information

Footnotes

1. Elyas AA, Patsalos PN, Agbato OA, Brett EM, Lascelles PT. Factors influencing simultaneous concentrations of total and free carbamazepine and carbamazepine-10,11-epoxide in serum of children with epilepsy. Ther Drug Monit. 1986; 8(3):288-292. 3750372
2. Spina E, Perucca E. Clinical significance of pharmacokinetic interactions between antiepileptic and psychotropic drugs. Epilepsia. 2002; 43(Suppl 2):37-44. 11903482
3. Engel J, Seizures and Epilepsy. Contemporary Neurology Series. Philadelphia, Pa: FA Davis Co;1989.

References

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

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
007419 Carbamazepine(Tegretol),S 3432-2 007422 Carbamazepine(Tegretol), S ug/mL 3432-2
Reflex Table for Carbamazepine(Tegretol), S
Order Code Order Name Result Code Result Name UofM Result LOINC
Reflex 1 007000 See below: 007000 See below: N/A

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