A 42-year-old male with diabetes mellitus (DM) for 5 years, on insulin therapy for 2 years (average 32-40U daily). After consuming refrigerated food 2 days ago, he developed watery diarrhea (4-6 episodes/day), somnolence for 1 day, and intermittent convulsions for 2 hours, prompting emergency admission. Physical exam: dehydrated appearance, light coma, BP 16/10 kPa, HR 120/min. Which test is most helpful for diagnosing the coma?