HbA1c (glycated hemoglobin) is a blood test that measures the average blood glucose level over the past 2-3 months. When glucose circulates in the bloodstream, it binds to hemoglobin in red blood cells, forming glycated hemoglobin. Since red blood cells live approximately 120 days, HbA1c provides a reliable long-term indicator of glycemic control. It is the primary tool used by healthcare providers to diagnose diabetes, monitor treatment effectiveness, and assess the risk of diabetes-related complications. Unlike fasting glucose tests, HbA1c does not require fasting and can be measured at any time of day.
HbA1c can be estimated from average blood glucose using the formula: HbA1c (%) = (Average Glucose in mg/dL + 46.7) / 28.7. Conversely, estimated Average Glucose (eAG) can be calculated from HbA1c: eAG (mg/dL) = 28.7 x HbA1c - 46.7. These formulas are derived from the ADAG (A1C-Derived Average Glucose) study. You can also use our blood sugar converter to convert between glucose units.
HbA1c may be inaccurate in patients with hemoglobin variants (sickle cell disease, thalassemia), iron deficiency anemia, hemolytic anemia, or chronic kidney disease. Pregnancy, recent blood transfusions, and certain medications (such as erythropoietin) can also affect results. The test reflects an average and does not capture glucose variability or hypoglycemic episodes. Ethnic differences in glycation rates may influence results.
FAQ
What are the target HbA1c levels?
For most adults with diabetes, the American Diabetes Association recommends an HbA1c target below 7%. For healthy individuals, normal HbA1c is below 5.7%. Values between 5.7% and 6.4% indicate prediabetes, and 6.5% or higher indicates diabetes. Individual targets may vary based on age, comorbidities, and risk of hypoglycemia.
How often should HbA1c be tested?
For people with well-controlled diabetes, HbA1c testing is recommended every 6 months. For those not meeting glycemic targets or with recent treatment changes, testing every 3 months is advised. For screening purposes in at-risk individuals, annual testing may be appropriate. Consult your healthcare provider for personalized recommendations.
What does eAG (estimated Average Glucose) mean?
eAG translates HbA1c into average blood glucose units (mg/dL or mmol/L) that are more familiar from daily monitoring. For example, an HbA1c of 7% corresponds to an eAG of approximately 154 mg/dL (8.6 mmol/L). This helps patients better understand their long-term glucose control in relation to their daily glucose readings. Use our blood sugar converter for unit conversions.
Can HbA1c be used to diagnose diabetes?
Yes, HbA1c is one of the criteria for diagnosing diabetes. An HbA1c of 6.5% or higher on two separate occasions confirms a diabetes diagnosis. Values between 5.7% and 6.4% indicate prediabetes. However, HbA1c alone should not be used for diagnosis in conditions that affect red blood cell turnover or hemoglobin variants. A fasting glucose or oral glucose tolerance test may be needed for confirmation.