Estimated Average Glucose (eAG) and HbA1c Conversion: Evidence‑Based Guidance

Formula and Correlation

• The estimated average glucose (eAG) can be calculated from HbA1c using the formula eAG (mg/dL) = 28.7 × HbA1c (%) – 46.7; this relationship is derived from the international ADAG study, which demonstrated a strong linear correlation (r = 0.92) between HbA1c and mean glucose concentrations. 1

Standard Conversion Table (ADA)

Values in parentheses represent 95 % confidence intervals. 1

Evidence Base (ADAG Study)

• The ADAG study analyzed approximately 2,700 glucose measurements collected over three months per HbA1c value in 507 adults spanning type 1 diabetes, type 2 diabetes, and individuals without diabetes, providing the empirical foundation for the eAG formula and conversion table. 1

Guideline Recommendations for Reporting

• Because the correlation between HbA1c and average glucose is strong (r = 0.92), the American Diabetes Association (ADA) and the American Association for Clinical Chemistry (AACC) recommend that laboratory reports present both the HbA1c value and the corresponding eAG to aid patient understanding. 1

Limitations and Interfering Conditions

• Red‑blood‑cell turnover disorders (e.g., hemolytic anemia, other anemias, glucose‑6‑phosphate dehydrogenase deficiency, recent blood transfusion, erythropoiesis‑stimulating agents, end‑stage kidney disease, pregnancy) can render eAG calculations inaccurate. 1
• Hemoglobin variants may interfere with some HbA1c assays; however, most U.S. assays remain accurate for heterozygous common variants. 1
• In sickle‑cell disease (HbSS) or other homozygous hemoglobin variants, HbA1c cannot be measured because the individuals lack measurable HbA. 1

What eAG Does Not Convey

• eAG reflects only the average glucose concentration and provides no information about glycemic variability, hypoglycemia risk, or glucose excursions. 1

Racial and Ethnic Considerations

• African‑American individuals may exhibit slightly higher HbA1c values than non‑Hispanic White individuals for the same mean glucose level, an observation that remains under investigation. 2
• Afro‑Caribbean populations have shown lower HbA1c relative to glucose in some studies. 2

Applicability to Modern CGM Systems

• The ADAG data were collected in 2008 using early continuous glucose monitoring (CGM) systems combined with capillary glucose measurements; therefore, the derived eAG estimates may not fully reflect the performance of contemporary CGM technologies, which differ in calibration, accuracy, interstitial‑venous lag time, and wear patterns. 1

Alternative Monitoring When eAG Is Unreliable

• In situations where HbA1c interpretation is compromised (e.g., interfering conditions above), clinicians should consider self‑monitoring of blood glucose (SMBG), modern CGM, or glycated serum protein assays such as fructosamine or glycated albumin. 1
• For patients with significant glycemic variability, particularly those with type 1 diabetes, fructosamine or glycated albumin may provide a more useful assessment of short‑term glycemic control than HbA1c/eAG. 1

Estimated Average Glucose Calculation and Clinical Utility

Introduction to eAG

• The American Diabetes Association recommends using Estimated Average Glucose (eAG) to translate HbA1c results into average blood glucose levels, making it easier for patients to understand their glucose control over the preceding 2-3 months 3, 4.

Mathematical Relationship and Calculation

• eAG is calculated using the regression equation established by the international A1C-Derived Average Glucose (ADAG) study: eAG (mg/dL) = 28.7 × HbA1c (%) - 46.7, with a strong correlation coefficient of r = 0.92 between HbA1c and mean glucose 3, 4, 5.
• The ADAG study analyzed approximately 2,700 glucose measurements over 3 months per HbA1c measurement in 507 adults with type 1 diabetes, type 2 diabetes, and no diabetes 3, 4, 5.

Standard Conversion Table

• The following table shows the correlation between HbA1c percentages and eAG values:

• Values in parentheses represent 95% confidence intervals 3, 4.

Clinical Purpose and Utility

• eAG uses the same measurement units (mg/dL or mmol/L) that patients see on their glucose meters and lab reports, making HbA1c results more intuitive and easier to understand 6, 7, 8, 9.
• The American Diabetes Association and the American Association for Clinical Chemistry determined that the correlation in the ADAG trial is strong enough to justify reporting both the A1C result and the eAG result when a clinician orders the A1C test 3, 5.

Critical Limitations and When NOT to Use eAG

• eAG reflects only average glucose levels and provides no information about glycemic variability, hypoglycemia risk, or extreme blood glucose readings 6, 7, 8, 9.
• Conditions affecting red blood cell turnover, such as hemolytic anemia, other anemias, glucose-6-phosphate dehydrogenase deficiency, recent blood transfusion, use of erythropoiesis-stimulating drugs, end-stage kidney disease, and pregnancy, make eAG unreliable 3, 4, 5.

Relationship to Treatment Goals

• The American Diabetes Association recommends keeping A1C levels below 7% in most patients, which corresponds to an eAG below 154 mg/dL 6, 7, 8, 9.
• This target aims to reduce long-term complications but does not indicate compliance with therapy or predict acute complications 6, 7, 8, 9.