Paediatricscommon
A 15-year-old girl is brought to the emergency department by her mother two hours after swallowing a handful of tablets during an argument. She says she took only her mother's sertraline and nothing else, and now regrets it. She is alert and tearful, with normal observations and no tremor, sweating or clonus. Her electrocardiogram is normal. Which one of the following investigations is most likely to reveal a lethal poisoning she has not disclosed?