A 71-year-old woman with a 5-year history of diabetes mellitus (DM) presents with cough, productive sputum, and fever for 1 week, somnolence for 2 days, and coma for 5 hours. Physical examination reveals moderate coma, dry skin, respiratory rate of 24/min, bilateral pulmonary rales, and heart rate of 120/min. Which test would be most helpful for diagnosis at this time?