What analyte is monitored in treatment of seizures?

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Therapeutic drug monitoring focuses on measuring the drug concentration in the blood to keep treatment effective while avoiding toxicity. Phenytoin is the best example here because it has saturable, non-linear pharmacokinetics; small dose changes can cause large shifts in serum levels, so knowing the actual concentration is essential for safe and effective dosing. The typical target range is about 10–20 μg/mL, with toxicity signs like nystagmus and ataxia appearing as levels rise, while too-low levels may fail to control seizures. Because phenytoin binds to albumin, total levels can be misleading in conditions that alter albumin, so free (unbound) levels may be necessary in some patients. Other antiepileptic drugs such as carbamazepine, valproate, and lamotrigine are monitored in certain contexts, but phenytoin’s narrow therapeutic window and nonlinear kinetics make its serum concentration the most important analyte to track in seizure treatment.