What laboratory test is essential for monitoring a client receiving phenytoin for potential side effects?

Phenytoin is an anticonvulsant medication used primarily in the management of seizures. One of the key concerns when a patient is on phenytoin therapy is the potential development of drug-induced hematologic side effects, such as leukopenia or thrombocytopenia. Monitoring a complete blood count (CBC) is critical because it provides information on multiple blood components, including red blood cells, white blood cells, and platelets.

Given the risk of blood dyscrasias associated with phenytoin, the CBC serves as a comprehensive assessment tool to identify any reductions in white blood cells which could signal an increased risk for infections or issues with blood cell production. This is particularly important because some of the side effects related to phenytoin, such as megaloblastic anemia, directly affect the components measured in a CBC.

While liver function tests are also important for monitoring patients on phenytoin due to its metabolism through the liver, the CBC is specifically essential for early detection of potential hematologic complications. Therefore, the complete blood count is the laboratory test that plays a critical role in ensuring patient safety during phenytoin therapy.