Importance of Renal Function Testing Before MRI

A nurse is caring for a patient who is scheduled for an MRI of the brain to assess for possible lesions related to neurological symptoms. The patient has a history of claustrophobia and expresses anxiety about being in the tunnel of the MRI machine. In preparing the patient, the nurse recognizes various diagnostic measures should be ordered prior to the MRI to ensure safety and effectiveness. Which of the following tests is MOST important for the nurse to evaluate before the MRI can be safely performed?