A 34-year-old woman with poorly controlled type 1 diabetes mellitus presents to the emergency department with altered mental status, rapid breathing, and fruity-smelling breath. Arterial blood gas shows pH 7.22, HCO₃⁻ 12 mEq/L, and PaCO₂ 28 mmHg. Which investigation is most specific for confirming the diagnosis of diabetic ketoacidosis?