What HbA1c measures
HbA1c is the fraction of hemoglobin that has glucose attached. Because the attachment is irreversible and red blood cells live about three months, HbA1c integrates glucose exposure over that period. It does not show day-to-day variation, so a normal HbA1c can coexist with dangerous swings. This is why it is used alongside, not instead of, direct glucose measurement.
Why control matters
Persistent hyperglycemia injures small vessels and nerves, causing retinopathy, nephropathy, and neuropathy, and it accelerates atherosclerosis. These complications are the reason treatment targets exist. Acute problems such as hypoglycemia and ketoacidosis are tied to the treatment itself and to insulin deficiency, so monitoring must detect both the chronic trend and the acute event.