A 55-year-old man with a history of type 2 diabetes mellitus (T2DM) presents for a routine check-up. He reports no recent hypoglycemic episodes but admits to occasional dietary noncompliance. His fasting blood glucose is 110 mg/dL (normal: 70–99 mg/dL). The physician orders a glycated hemoglobin (HbA1c) test to assess long-term glucose control. Which of the following best describes what HbA1c measures?
