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Lab Test: Carbamazepine (Blood) Level

    Lab Test
    • Carbamazepine (Tegretol) blood measurement
    • Measurement of carbamazepine levels in serum or plasma to facilitate therapeutic or toxicity monitoring.
    Reference Range
    • 4-12 mcg/mL (17-51 micromol/L)
    • Toxicity:
      • Adults:  > 15 mcg/mL (> 63 micromol/L)
      • Cardiac Effects:  dysrhythmias and conduction defects may occur at levels as low as 3.2 mg/L
      • Neurologic Effects: occur at concentrations of 4 to 5 times the upper therapeutic limit and include (nystagmus, ataxia, gross intention tremor, dysarthris, respiratory depression, drowsiness, stupor, or coma)
    Indications & Uses
    • Seizures: Drug level monitoring during carbamazepine (Tegretol) therapy
    • Suspected carbamazepine toxicity
    Clinical Application
    • Carbamazepine blood levels should be reviewed within the context of clinical findings, such as a change in seizure frequency. 
    • Serum concentrations of carbamazepine may be affected by the co-administration of other antiepileptic drugs and by the age of the patient.
    • Seizure protection is best assessed in the context of trough levels, and toxicity potential is best assessed in the context of peak levels.
    • At the onset of treatment, carbamazepine levels initially rise, followed by a slow decrease over the next 3 to 4 weeks due to auto-activation of metabolic enzymes (e.g., CYP3A4) involved in eliminating carbamazepine.  This is due to its own 10,11-epoxide metabolite and usually indicates the need for a dosage increase.
    • Sub-therapeutic carbamazepine levels may indicate patient noncompliance or an interaction with another substance in the serum sample.
    • Blood concentrations can change when switching formulations or brands of carbamazepine.
    • Grapefruit can inhibit the metabolism of carbamazepine due to inhibition of CYP3A4, thereby increasing the levels of carbamazepine.
    Related Tests
    • Phenytoin levels


    Drug-Lab Interactions
    • None
    Test Tube Needed
    • Red top tube (with no additives)
    • Heparin (Green top tube)
    • EDTA tube (Lavender top)
    • Draw serum or plasma sample ~ 1mL
    • If using serum separator tube, fill tube completely and process promptly.
    • Collect trough sample.
    • Collect at a consistent time of day.
    • Avoid hemolysis.
    Storage and Handling
    • May store at room temperature for several hours.
    • May store at -20°C for up to 1 year.
    What To Tell Patient Before & After
    • None
    • Elyas AA et al. Factors influencing simultaneous concentrations of total and free carbamazepine and carbamazepine-10,11-epoxid in serum of children with epilepsy.  Ther Drug Monit 1986;8(3):288-92.
    • Arroyo S et al.  Carbmazepine in comparative trials: pharmacokinetic characteristics too often forgotten.  Neurology 1999;53:1170-4.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.

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MESH Terms & Keywords

  • Carbamazepine, Tegretol