A 28-year-old man with a history of alcoholism is brought to the hospital with severe hypoglycemia (blood glucose 35 mg/dL), metabolic acidosis (pH 7.18, HCO₃⁻ 10 mEq/L), and elevated serum lactate. You suspect alcoholic ketoacidosis superimposed on lactic acidosis. Which investigation would best differentiate the ketotic component from the lactic acidosis component?