A 72-year-old man with a long history of type 2 diabetes treated with insulin visits his family physician for a routine checkup. He states that he administers his insulin just as the doctor ordered,...

A 72-year-old man with a long history of type 2 diabetes treated with insulin visits his family physician for a routine checkup. He states that he administers his insulin just as the doctor ordered, but continues to present with random blood sugar levels well above 200 mg/dL. The doctor suspects the patient has not been using his insulin appropriately for some time now. What is the best way to measure long-term diabetic control of blood glucose?

A. Glucose tolerance test

B. Fasting serum glucose

C. HbA1c

D. Urinalysis

E. CBC