In a patient undergoing treatment with phenytoin, what laboratory finding indicates toxicity?

• A. 20 mcg/mL
• B. 38 mcg/mL
• C. 15 mcg/mL
• D. 25 mcg/mL

Answer: (B)

Phenytoin is an anticonvulsant medication used primarily in the management of seizures. The therapeutic range for phenytoin plasma concentration is typically between 10 to 20 mcg/mL. Values above this range can indicate potential toxicity, which may lead to various neurological symptoms and adverse effects.

In the context of the provided options, a serum concentration of 38 mcg/mL is clearly above the established therapeutic range. This significant elevation is indicative of toxicity and can lead to symptoms such as nystagmus, ataxia, confusion, and even more severe neurologic impairment. Recognizing this value as an indicator of toxicity is critical in clinical practice, as it necessitates prompt adjustment of the medication or other interventions to mitigate the risks associated with phenytoin overdose.

The other concentrations provided fall either within or just near the therapeutic range, which is not indicative of toxicity. Understanding these thresholds is essential for safe medication management in patients receiving phenytoin.