Why is monitoring laboratory values needed for a patient taking phenytoin?

• A. To ensure the drug level is within the therapeutic range.
• B. To monitor liver function only.
• C. To assess nutrition status.
• D. To check for electrolyte balance.

Answer: (A)

Phenytoin requires therapeutic drug monitoring because its blood concentration must stay within a narrow range due to variable pharmacokinetics. Even small changes in dose, interactions with other drugs, or conditions that affect protein binding (like low albumin) can cause big shifts in levels. Regular lab measurements of the serum concentration guide dose adjustments to maintain seizure control while avoiding toxicity. High levels risk toxicity (ataxia, nystagmus, confusion), while low levels can lead to breakthrough seizures. While liver function and nutrition can influence levels, the main purpose of monitoring is to ensure the drug concentration stays within the therapeutic window, with free levels considered if protein binding is altered.