Hemoglobin (Hb) A1c

The Hemoglobin A1c test (glycated hemoglobin, glycohemoglobin) is used to assess chronic glycemic control in people with diabetes. It helps determine whether current therapy is achieving targets and informs treatment adjustments, and it can reveal sustained hyperglycemia in those newly diagnosed. When applied with appropriate clinical and laboratory data, HbA1c supports the diagnosis of diabetes mellitus and the identification of individuals with prediabetes.

HbA1c reflects a time‑weighted average of glycemia and does not capture acute changes or within‑day variability. Interpretation requires consideration of factors that alter red blood cell turnover or hemoglobin composition. Shortened erythrocyte survival—such as in hemolytic anemia, acute or chronic blood loss, or pregnancy—can produce HbA1c values that are lower than the true ambient glycemia. Hemoglobin variants may affect red cell lifespan or in vivo glycation. Marked discordance between HbA1c and observed plasma glucose should prompt evaluation for a hemoglobin variant and the use of alternative glucose‑based testing in accordance with ADA guidance. Elevated fetal hemoglobin (HbF >10%) can yield lower‑than‑expected HbA1c results with some methods.