Which laboratory parameter should be monitored closely in a client undergoing phenytoin therapy?

In the context of a client undergoing phenytoin therapy, monitoring the complete blood count (CBC) is essential due to the risk of adverse effects associated with the medication. Phenytoin can lead to hematological complications, such as leukopenia, thrombocytopenia, and even agranulocytosis. By regularly checking the CBC, healthcare providers can identify any significant drops in blood cell counts early and take appropriate action to mitigate risks to the patient, such as increased susceptibility to infections or bleeding.

Liver function tests are important in the overall monitoring of a patient's health, especially since phenytoin is metabolized in the liver, but they are not as critical for immediate effects as the CBC. Monitoring potassium levels is more relevant in cardiac care or with other medications that might cause an electrolyte imbalance rather than specifically phenytoin therapy. BUN and creatinine levels can be important for assessing kidney function, but they are less directly related to the potential side effects of phenytoin than the CBC. Thus, the focus on the complete blood count in this scenario is crucial for maintaining patient safety and identifying medication-related issues promptly.