Lab Test: Carbamazepine (Blood) Level
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- Measurement of carbamazepine levels in serum or plasma to facilitate therapeutic or toxicity monitoring.
- 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)
- Seizures: Drug level monitoring during carbamazepine (Tegretol) therapy
- Suspected carbamazepine toxicity
- 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.
- 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.
- May store at room temperature for several hours.
- May store at -20°C for up to 1 year.
- 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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Indications & Uses
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