What is the best initial laboratory test to recommend for a patient presenting with a seizure and significant fever?

In a patient presenting with a seizure accompanied by significant fever, the clinical context often suggests the possibility of a viral infection, such as influenza, especially in pediatric patients. Rapid testing for influenza is particularly valuable in this scenario because it can quickly confirm or rule out influenza as the cause of the febrile seizure. This can help guide subsequent management, including the decision to initiate antiviral therapy if influenza is diagnosed and if indicated, especially in cases with severe symptoms.

Other tests such as a CBC with differential or electrolyte panel may provide some helpful information about infection or metabolic imbalances, but they are not as directly relevant to the immediate concern of identifying a potentially treatable cause of the fever and seizure. Liver function tests are generally not prioritized in the acute setting of a febrile seizure unless there are specific signs or symptoms suggesting hepatobiliary dysfunction.

Therefore, the initial focus should be on identifying whether a viral infection like influenza is present, as this can have significant implications for treatment and further evaluation.