Delayed elevation in carbamazepine concentrations after overdose: a retrospective poison center study

An initial carbamazepine concentration may initially be supratherapeutic, therapeutic, or even subtherapeutic only to persist to rise over time. The aim of this study was to report the frequency of toxic carbamazepine concentrations continuing to rise and to estimate how often an initially therapeutic or subtherapeutic concentration misrepresents the potential toxicity of an acute carbamazepine overdose. An 8-year retrospective search of all carbamazepine exposures reported to the Illinois Poison Center (January 1, 2001 through December 31, 2008) was reviewed. Inclusion criteria were acute poisonings with a documented carbamazepine concentration of >12 μg/mL at any time. Those with initial concentrations of >12 μg/mL that subsequently increased over time were recorded. Additionally, those cases that initially had therapeutic (4-12 μg/mL) or subtherapeutic (<4 μg/mL) concentration were identified. Descriptive statistics were used to analyze the data. A total of 1424 cases were reported. Of the 523 patients with documented concentrations of >12 μg/mL, 93 patients (17.8%) had initial carbamazepine concentrations >12 μg/mL and continued to rise. Sixteen patients (3.5%) had initial carbamazepine concentrations that were therapeutic (4-12 μg/mL) and 7 patients (1.3%) had initial carbamazepine concentrations <4 μg/mL before rising >12 μg/mL. Certain patients had progressive decreases in level of consciousness corresponding to increasing carbamazepine concentrations. Additionally, several patients with initial levels of therapeutic or subtherapeutic concentration later became comatose and required ventilator management. Initial serum carbamazepine concentrations can be misleading. Serial measurements documenting a declining carbamazepine concentration or prolonged observation are recommended when managing these overdoses.