55 year old women with muscle weakness for a month. She is unable to comp her hair or raise her arm above her shoulder. Her symptoms fluctuant through the day depends on her activity level. She gets double vision at night. Clinically she has bilateral mild ptosis gets worse when gaze upward. Full oculomotor muscle movement with no diplopia. Her shoulder abductor 4/5 power and the rest of upper arms and lower arms is 5/5. What is the diagnostic test?