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Carbamazepine is an anticonvulsant medication effective in the treatment of complex partial seizures, with or without generalization to tonic-clonic seizures. It is frequently administered in conjunction with other anti-epileptic medications, such as phenytoin and valproic acid. In seizure control, serum trough levels are useful to assess; adequacy of initial dosage, patient compliance, changes in regimen or physiologic state, or confirmation of intoxication. The half-life of carbamazepine decreases approximately 30 50% with multiple doses. Phenytoin or phenobarbital may decrease serum levels. A steady state concentration is reached 2 to 4 weeks after initiating therapy. A trough level should be drawn immediately before the next dose. The therapeutic range is 4 - 12 ug/mL as measured by fluorescence polarization immunoassay. Toxicity may occur in the 8.0 12.0 ug/mL range when the patient is also taking other anti epileptic drugs. Generally, toxicity is not seen until levels exceed 12 ug/mL. The critical value is >20 ug/mL. Circulating carbamazepine is 75% protein bound under normal circumstances. In severe uremia, carbamazepine may be displaced from protein resulting in a higher free fraction of circulating drug. Since neurologic activity and toxicity are directly related to the free fraction, determination of free carbamazepine levels may be useful in adjusting dosage for uremic patients. The therapeutic range for free carbamazepine is 0.5 - 4.0 ug/mL. Levels >4.0 ug/mL may be associated with toxicity, even though total carbamazepine levels are < 12 ug/mL. Specimen requirement is serum collected in a plain red top tube. |
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