A 38-year-old woman presents with polyuria, polydipsia, and weight loss for 3 weeks. Fasting blood glucose is 285 mg/dL and venous pH is 7.28 with bicarbonate of 14 mEq/L. She is clinically euvolemic with no ketonuria on dipstick. Which is the most appropriate investigation to confirm the diagnosis of diabetic ketoacidosis?