A 62-year-old woman with heart failure and preserved ejection fraction (HFpEF) is prescribed a thiazide diuretic for hypertension and fluid management. After 3 weeks, she reports polyuria, polydipsia, and fatigue. Fasting blood glucose is 180 mg/dL (previously 95 mg/dL). Which investigation would be most appropriate to confirm the metabolic complication and assess its severity?