HbA1c reflects average blood glucose over the past 2–3 months by measuring the percentage of hemoglobin bound to glucose. An A1C of 6.5% corresponds to an estimated average glucose (eAG) of about 140 mg/dL, calculated as A1C × 28.7 − 46.7. Enter an A1C percentage to get the eAG, or enter an eAG to find the corresponding A1C. Results are classified into normal (<5.7%), prediabetes (5.7–6.4%), or diabetes (≥6.5%) based on ADA thresholds.
The A1C to eAG Formula
The ADAG study established the conversion: eAG (mg/dL) = 28.7 × A1C − 46.7. This linear formula was derived from continuous glucose monitoring data across a diverse population. The reverse is A1C = (eAG + 46.7) / 28.7. For the IFCC standard in mmol/mol, the formula is (A1C% − 2.15) × 10.929.
ADA Diagnostic Thresholds
The American Diabetes Association classifies A1C results into three ranges. Below 5.7% is normal. Between 5.7% and 6.4% indicates prediabetes — elevated risk that may be reversible with lifestyle changes. At 6.5% or above, the result meets the diagnostic criterion for type 2 diabetes. These thresholds apply to standard lab assays; point-of-care devices may have wider margins.
Factors That Affect A1C Accuracy
Conditions that alter red blood cell lifespan can skew A1C readings. Iron deficiency anemia, hemoglobin variants (HbS, HbC, HbE), chronic kidney disease, and recent blood transfusions may produce misleadingly high or low results. In these cases, fructosamine or glycated albumin tests provide an alternative 2–3 week glucose average that is not affected by hemoglobin abnormalities.
FAQ
Q: What A1C level is considered diabetic?
A: An A1C of 6.5% or higher on two separate tests meets the ADA diagnostic criterion for diabetes. A single result should be confirmed with a repeat test or a fasting glucose test.
Q: How often should A1C be tested?
A: Twice a year for people with stable glucose control. Every three months for those adjusting treatment or not meeting targets. The test reflects a 2–3 month average, so more frequent testing does not add information.
Q: Can A1C be lowered without medication?
A: In the prediabetes range, studies show that dietary changes, regular exercise, and modest weight loss (5–7% of body weight) can reduce A1C by 0.5–1.0 percentage points. At higher levels, medication is usually needed alongside lifestyle changes.