A 38-year-old woman with diabetes and hypertension presents with eruptive xanthomas on her buttocks and elbows. Fasting lipid profile: total cholesterol 320 mg/dL, LDL-C 140 mg/dL, HDL-C 28 mg/dL, triglycerides 1200 mg/dL. Serum is visibly lipemic. Which investigation is most appropriate to confirm the underlying lipid disorder?