After a child on phenytoin shows signs of toxicity, what is the recommended next step?

In the context of monitoring phenytoin toxicity, measuring the concentration of free phenytoin in the blood is critical. Phenytoin is highly protein-bound, primarily to albumin, and the total phenytoin level can sometimes be misleading due to changes in protein levels, such as those that may occur in children or under various clinical conditions. When a child exhibits signs of toxicity, it is essential to assess the unbound fraction of the drug, which is responsible for pharmacological activity and toxicity.

Measuring free phenytoin allows healthcare providers to obtain a more accurate picture of the phenytoin's effect in the body, particularly in cases where altered protein binding may affect total levels. This is especially important in pediatric patients who may have variable protein levels compared to adults.

Given this context, the recommendation to measure free phenytoin using the last specimen directly addresses the concern for assessing the child's true exposure to the drug and the associated risk of toxicity. This step enables appropriate clinical decision-making and potential adjustments to therapy based on accurate pharmacokinetic understanding.