What is the difference between estimated Average Glucose (eAG) and Hemoglobin A1C (A1C)?

Difference Between eAG and A1C

eAG (estimated Average Glucose) is simply a mathematical translation of A1C into the same units (mg/dL or mmol/L) that patients see on their glucose meters, while A1C is the actual laboratory measurement of glycated hemoglobin expressed as a percentage. 1

What A1C Actually Measures

• A1C is a laboratory test that measures the percentage of hemoglobin proteins in red blood cells that have glucose attached to them over the preceding 2-3 months 2
• A1C is reported as a percentage (e.g., 7%, 8%, 9%) and directly reflects chronic glycemic exposure 1
• The test is an indirect measure of average glycemia based on red blood cell turnover 1

What eAG Represents

• eAG is a calculated value derived from A1C using the regression equation: eAG (mg/dL) = 28.7 × A1C (%) - 46.7 3, 4
• This equation was established by the international A1C-Derived Average Glucose (ADAG) study, which analyzed approximately 2,700 glucose measurements over 3 months per A1C measurement in 507 adults 1, 3
• The correlation between A1C and average glucose was r = 0.92, which the American Diabetes Association and American Association for Clinical Chemistry deemed strong enough to justify reporting both values together 1, 3

Standard Conversion Values

The American Diabetes Association provides the following correlation table 1:

• A1C 5% = eAG 97 mg/dL (5.4 mmol/L)
• A1C 6% = eAG 126 mg/dL (7.0 mmol/L)
• A1C 7% = eAG 154 mg/dL (8.6 mmol/L)
• A1C 8% = eAG 183 mg/dL (10.2 mmol/L)
• A1C 9% = eAG 212 mg/dL (11.8 mmol/L)
• A1C 10% = eAG 240 mg/dL (13.4 mmol/L)

Why eAG Was Created

• The primary purpose of eAG is to make A1C results more intuitive for patients by expressing them in the same measurement units (mg/dL or mmol/L) they see daily on their glucose meters 3, 2
• This translation was intended to improve patient understanding and communication between patients and healthcare providers 3

Critical Shared Limitations

Both A1C and eAG have identical limitations because eAG is simply derived from A1C 3:

• Neither provides information about glycemic variability, hypoglycemia risk, or extreme glucose excursions 1, 3
• Both are unreliable in conditions affecting red blood cell turnover: hemolytic anemia, G6PD deficiency, recent blood transfusion, erythropoiesis-stimulating drugs, end-stage kidney disease, and pregnancy 1, 5
• Hemoglobin variants can affect both measurements equally 1

Important Clinical Pitfall

Research has demonstrated frequent and clinically significant disagreement between eAG and actual mean blood glucose (MBG) from patient glucose monitoring 6, 7:

• eAG over or underestimated actual MBG by ≥28.7 mg/dL (equivalent to A1C difference of ≥1%) in approximately 33% of patients 6
• eAG tends to overestimate MBG at lower glucose levels and may lead to inappropriate intensification of therapy with increased hypoglycemia risk 6, 7
• Biological variation in hemoglobin glycation (measured by hemoglobin glycation index) causes systematic bias, with eAG underestimating MBG in low-glycation patients and overestimating in high-glycation patients 7

When to Use Each Value

• Use A1C as the primary laboratory measure for diagnosis, monitoring, and treatment decisions 1
• Report eAG alongside A1C when ordering the test to help patients relate the result to their daily glucose monitoring 1, 3
• When eAG and patient's downloaded meter average glucose are discordant, trust the actual glucose measurements and investigate for conditions affecting red blood cell turnover or hemoglobin variants 5, 6, 7
• For patients with glycemic variability (type 1 diabetes or severe insulin deficiency in type 2 diabetes), combine blood glucose monitoring/CGM results with A1C rather than relying on eAG alone 1

Practical Algorithm for Discordant Values

When A1C/eAG appears inconsistent with patient's glucose monitoring 5:

1. Obtain complete blood count to assess for anemia or red blood cell abnormalities
2. Review medication list for erythropoiesis-stimulating agents
3. Consider hemoglobin electrophoresis if variant suspected
4. Switch to plasma glucose criteria for diagnosis and monitoring if red blood cell conditions are present