Formula to Estimate Average Glucose from HbA1c
The formula to estimate average glucose from HbA1c is: Average Glucose (mg/dL) = (28.7 × HbA1c) - 46.7, or alternatively, you can use the simplified conversion table where HbA1c of 6% = 126 mg/dL, 7% = 154 mg/dL, 8% = 183 mg/dL, and so on, with each 1% increase in HbA1c corresponding to approximately 29 mg/dL increase in average glucose. 1
Standard Conversion Values
The American Diabetes Association endorses the following correlation between HbA1c and mean plasma glucose based on the ADAG (A1C-Derived Average Glucose) study, which included approximately 2,700 glucose measurements over 3 months per HbA1c measurement 1:
- HbA1c 6% = Mean glucose 126 mg/dL (7.0 mmol/L) 1
- HbA1c 7% = Mean glucose 154 mg/dL (8.6 mmol/L) 1
- HbA1c 8% = Mean glucose 183 mg/dL (10.2 mmol/L) 1
- HbA1c 9% = Mean glucose 212 mg/dL (11.8 mmol/L) 1
- HbA1c 10% = Mean glucose 240 mg/dL (13.4 mmol/L) 1
- HbA1c 11% = Mean glucose 269 mg/dL (14.9 mmol/L) 1
- HbA1c 12% = Mean glucose 298 mg/dL (16.5 mmol/L) 1
Estimated Average Glucose (eAG) Reporting
The American Diabetes Association and American Association of Clinical Chemistry determined that the correlation (r = 0.92) between HbA1c and average glucose is strong enough to justify reporting both the HbA1c result and an estimated average glucose (eAG) result when ordering the test 1. A calculator for converting HbA1c to eAG in either mg/dL or mmol/L is available through professional diabetes resources 1.
Important Clinical Caveats
HbA1c reflects average glycemia over approximately 2-3 months (the lifespan of red blood cells), not real-time glucose levels. 1, 2, 3 This means:
- The formula provides population-level estimates, not precise individual values - there can be significant variation between measured average glucose and HbA1c-derived estimates in individual patients 1, 4
- HbA1c does not capture glycemic variability or hypoglycemic episodes - two patients with identical HbA1c values may have vastly different glucose patterns, with one experiencing frequent hypoglycemia and hyperglycemia while the other maintains stable glucose levels 1, 4
Conditions That Invalidate the Formula
Do not use HbA1c to estimate average glucose in patients with conditions affecting red blood cell turnover, as these will produce falsely low or high HbA1c values that do not reflect true glycemic control 1, 2, 5:
- Hemolytic anemia - shortened RBC lifespan produces falsely low HbA1c 1, 2, 5
- Recent blood loss or transfusion - alters the age distribution of circulating RBCs 1, 2
- Erythropoietin therapy - increases proportion of young RBCs with less glycation 1
- Pregnancy (second and third trimesters) - increased RBC turnover 1
- Hemoglobin variants (sickle cell trait, other hemoglobinopathies) - may interfere with assay measurement 1
- End-stage kidney disease - affects both RBC turnover and assay accuracy 1
In these situations, use plasma glucose criteria exclusively for diagnosis and monitoring 1, 2.
Racial and Age Considerations
African Americans may have slightly higher HbA1c values than non-Hispanic whites for the same average glucose level, though the ADAG study showed no statistically significant differences, only trends 1. This remains an area requiring further research 1.
HbA1c increases with age in non-diabetic individuals - approximately 0.074% per decade of life, meaning a 70-year-old may have HbA1c 0.47% higher than a 30-year-old with identical glucose control 6. However, current diagnostic thresholds do not adjust for age 6.
Verification of Accuracy
Ensure HbA1c testing uses NGSP-certified methods standardized to the DCCT assay - only these methods produce results that correlate accurately with the conversion formulas above 1, 2. If there is marked discordance between measured HbA1c and plasma glucose levels, consider assay interference from hemoglobin variants and use an alternative assay or rely on plasma glucose measurements 1.