The Hemoglobin A1c (HbA1c) test provides a comprehensive picture of blood sugar management over time, but its result is commonly given as a percentage, which can feel abstract. This percentage represents a long-term measure of average blood sugar control, offering a view that daily fingerstick measurements cannot provide. Understanding what this percentage means in terms of familiar daily blood glucose readings is a common challenge. This article will explain the biological basis of the HbA1c test and detail the precise method used to translate this percentage result into the Estimated Average Glucose (eAG).
Understanding the HbA1c Measurement
The HbA1c test relies on glycation, a natural process where sugar molecules in the bloodstream permanently attach to hemoglobin inside red blood cells. Since red blood cells circulate in the body for approximately 120 days, the level of glycated hemoglobin reflects the average glucose concentration in the blood over their lifespan. The final result is weighted toward the most recent 30 days of glucose levels, as newer cells contribute a larger share to the overall percentage.
The test result is reported in one of two main unit systems. In the United States, the result is typically presented as a percentage, aligned with the National Glycohemoglobin Standardization Program (NGSP). This NGSP percentage is directly traceable to the landmark clinical trials that established the link between long-term blood sugar control and diabetes complications. Internationally, the result may be reported in millimoles per mole (mmol/mol), established by the International Federation of Clinical Chemistry (IFCC). While the two systems use different numerical scales, they measure the same biological reality and can be correlated using mathematical formulas.
Converting HbA1c to Estimated Average Glucose (eAG)
The Estimated Average Glucose (eAG) was developed to convert the HbA1c percentage into a more intuitive unit. The eAG is expressed in the same units used for daily self-monitoring: milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L). This conversion allows for a direct comparison between a long-term laboratory result and routine daily glucose readings.
The mathematical relationship between HbA1c and eAG was precisely defined by the A1c-Derived Average Glucose (ADAG) Study, which involved hundreds of participants. Researchers combined data from continuous glucose monitoring and frequent self-monitoring to establish a strong linear correlation between the two values. This study provided the formula now widely accepted and recommended by major health organizations.
To calculate eAG in milligrams per deciliter (mg/dL), the official formula derived from the ADAG study is used: eAG (mg/dL) = (28.7 \(\times\) HbA1c) – 46.7. For example, an HbA1c of 7% converts to an eAG of 154 mg/dL. This calculation transforms the abstract percentage into a number that directly reflects the average concentration of glucose in the plasma.
For countries using the international standard unit of millimoles per liter (mmol/L), the conversion is also standardized. The conversion table below highlights key HbA1c percentages and their corresponding eAG values in both common blood sugar units.
| HbA1c (%) | eAG (mg/dL) | eAG (mmol/L) |
| :—: | :—: | :—: |
| 6.0 | 126 | 7.0 |
| 7.0 | 154 | 8.6 |
| 8.0 | 183 | 10.1 |
| 9.0 | 212 | 11.8 |
This direct mathematical link ensures that the eAG provides a reliable estimate of the average glucose level over the preceding months.
Interpreting Conversion Results and Standard Ranges
The converted eAG number helps patients understand the clinical significance of a long-term HbA1c result. By presenting the average blood sugar in familiar units, it makes the goal of long-term glucose management more tangible. The eAG facilitates better conversations between patients and healthcare providers about how well a treatment plan is working over the span of two to three months.
Clinical guidelines establish specific ranges for HbA1c that are used for both diagnosis and management. A normal HbA1c level is considered to be below 5.7%, which corresponds to an eAG of less than 117 mg/dL. Values falling between 5.7% and 6.4% are classified as prediabetes, translating to an eAG range of 117 to 137 mg/dL.
A result of 6.5% or higher is the diagnostic threshold for diabetes, which corresponds to an eAG of 140 mg/dL or greater. For most adults already diagnosed with diabetes, the target for good management is an HbA1c less than 7%, or an eAG below 154 mg/dL. These targets are designed to minimize the risk of long-term diabetes-related complications.
The eAG is a calculated average and does not account for the peaks and valleys of daily blood sugar fluctuations. A person with an eAG of 154 mg/dL could still have periods of very high and very low blood sugar. Therefore, the eAG must be considered alongside daily self-monitoring to provide a complete picture of glucose control.