HbA1c measures the percentage of hemoglobin proteins in your blood that have glucose attached. Because red blood cells live about 120 days, HbA1c reflects your average blood glucose over the past 2–3 months. It is the primary metric for assessing long-term glycemic control.

ADA recommends <7.0% for most adults with diabetes. <6.5% may be appropriate for selected patients without significant hypoglycemia risk. <8.0% may be acceptable for elderly patients or those with multiple comorbidities. Non-diabetic normal is <5.7%.

The ADAG formula (eAG = 28.7 × A1c − 46.7) was derived from a large study correlating continuous glucose monitoring with A1c. It provides a good population-level estimate but individual variation exists. Some conditions (hemoglobin variants, anemia, kidney disease) can affect A1c accuracy.

For diabetic patients meeting goals: every 6 months. For patients not meeting goals or after treatment changes: every 3 months. For diagnosis/screening: fasting glucose or A1c can be used. Point-of-care A1c testing provides results within minutes.