Why does HbA1c reflect blood sugar levels over the past 3 months?

HbA1c reflects blood sugar over the past 3 months because red blood cells, which contain haemoglobin, live for about 90-120 days. During this time, glucose in the bloodstream binds to haemoglobin, forming glycated haemoglobin (HbA1c). The more glucose in the blood, the higher the percentage of HbA1c. Since red blood cells are continuously renewed, the test provides an average of blood sugar control over their lifespan. This long-term view is useful for monitoring diabetes and determining the effectiveness of a patient’s treatment plan.