What lab should you get with phenytoin, and why?

The appropriate lab test to obtain with phenytoin is a complete blood count (CBC) to monitor for agranulocytosis. Phenytoin, an antiepileptic medication, can induce bone marrow suppression, leading to a reduction in white blood cells, specifically neutrophils. This condition is known as agranulocytosis and can increase the risk of infections, making regular monitoring crucial for patients on long-term phenytoin therapy.

Monitoring a CBC allows healthcare providers to detect significant changes in white blood cell counts early, enabling timely intervention to mitigate this serious side effect. While other lab tests may be relevant in certain contexts related to phenytoin, such as liver function tests for potential hepatotoxicity or electrolyte panels for hydration status, they are not as directly associated with the risks posed by long-term phenytoin use as agranulocytosis. Thyroid function tests, while important in assessing drug interactions, especially with medications that alter thyroid hormone levels, are less pertinent specifically to the common adverse effects of phenytoin. Thus, focusing on CBC monitoring is crucial in managing the safety of patients taking phenytoin.