HbA1c (glycated hemoglobin) measures the percentage of hemoglobin molecules that have glucose attached to them. Because red blood cells live approximately 90–120 days, A1C reflects average blood glucose over roughly the past three months. It's a retrospective average — it tells you where you've been, not moment-to-moment glucose behavior.
The limitations of A1C matter clinically. Conditions that shorten red blood cell lifespan — hemolytic anemia, sickle cell trait, iron deficiency, recent blood transfusion — can artificially lower A1C readings. Conversely, anything that prolongs red cell survival can falsely elevate A1C. Hemoglobin variants (HbS, HbC) interfere with certain A1C assay methods. In these patients, fructosamine or continuous glucose monitoring provides more accurate assessment.
A1C of 7% corresponds roughly to an average glucose of 154 mg/dL. The conversion formula is: estimated average glucose (eAG) = 28.7 × A1C – 46.7. But averages hide patterns. A patient with A1C of 7% could be consistently at 154 mg/dL — or they could be swinging between 60 and 240 mg/dL, which carries entirely different risk profiles for complications and hypoglycemia.
For most adults with Type 2 diabetes, A1C <7% is the standard target. More stringent targets (<6.5%) may be appropriate in younger patients with short disease duration and no hypoglycemia risk. Less stringent targets (<8%) are appropriate in elderly patients, those with significant hypoglycemia risk, limited life expectancy, or advanced complications. Individualization is the principle — not universal protocol application.