A 56-year-old man with a 14-year history of diabetes mellitus presents with poor vision, peripheral vascular disease, and mild proteinuria. Which of the following is the best monitor of the control of blood sugar levels in this patient?

A specific fraction of glycosylated hemoglobin in circulating red blood cells (hemoglobin A1c) is measured routinely to monitor the overall degree of hyperglycemia that occurred during the preceding 6 to 8 weeks. Nonenzymatic glycosylation of hemoglobin is irreversible, and the level of hemoglobin A1c, therefore, serves as a marker for glycemic control. None of the other choices are quantitative measures of glucose levels.Diagnosis: Diabetes mellitus