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How does the HbA1c test show long-term blood glucose levels?

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An anatomical illustration of the human pancreas shows its location near the stomach and intestines, a detailed cross-section highlighting ducts and the islet of Langerhans, and an inset of pancreatic tissue histology with labeled alpha, beta, and delta cells. — diabetes mellitus The islets of Langerhans are responsible for the endocrine function of the pancreas. Each islet contains beta, alpha, delta, and pancreatic polypeptide (PP) cells that are responsible for the secretion of pancreatic hormones. Beta cells secrete insulin, a well-characterized hormone that plays an important role in regulating glucose metabolism.

The HbA1c test, also known as glycated hemoglobin, measures the amount of glucose that has chemically bonded to hemoglobin A in the blood.12

Because glucose molecules in the bloodstream naturally attach themselves to hemoglobin—the protein responsible for carrying oxygen—the resulting glycohemoglobin serves as a stable record of blood sugar concentrations. Since red blood cells typically circulate for several months, this test provides a reliable average of a person's blood glucose levels over the preceding two to three months.134

In the context of managing diabetes mellitus, this measurement is a critical tool for assessing long-term glycemic control, as elevated levels indicate that blood sugar has not been adequately managed over that period.2