A 58-year-old man has experienced worsening double vision and eyelid drooping, particularly toward the end of the day, for 1 month. He also has had difficulty chewing his food at dinner. He was diagnosed with Sjogren syndrome more than a decade ago. On physical examination, he has 5/5 motor strength in his extremities that decreases to 4/5 strength with repetitive movement. Administration of edrophonium restores muscle strength. There is no pain on palpation and no decrease in joint mobility. Which of the following laboratory findings is most likely to be reported for this patient?