A 40-year-old woman is seen for a follow-up evaluation of multiple sclerosis, which was diagnosed 1 month ago. The patient reports feeling generally well. She has no significant medical history and does not drink or smoke. Her only medication is interferon beta-1a, which she has been taking since diagnosis and tolerates well.

On physical examination, vital signs are normal. Neurologic examination reveals internuclear ophthalmoplegia, right arm weakness, and mild ataxia, which were present at diagnosis and have remained stable. All other physical examination findings are unremarkable.

Which of the following is indicated for periodic monitoring of this patient's disease-modifying therapy?