The measurement of blood sugar involves two distinct values: a real-time glucose reading and a long-term average. Daily blood sugar levels are typically measured using a home meter, providing a snapshot of glucose concentration in milligrams per deciliter (mg/dL). The A1C is a percentage reflecting average blood sugar control over a much longer time frame. Medical science has established a standard method to translate the A1C percentage into a familiar glucose reading, helping individuals and healthcare providers understand long-term control using the same daily units.
Understanding Glucose vs. Hemoglobin Measurement
Daily blood sugar measurement and the A1C test differ fundamentally in what they analyze and the time period they represent. A standard blood glucose test measures the concentration of glucose circulating in the bloodstream at a specific instant. This reading is highly dynamic, fluctuating rapidly based on recent food intake, physical activity, and medication timing, providing immediate feedback for day-to-day decisions.
The A1C test measures the percentage of hemoglobin in red blood cells that has become “glycated,” meaning glucose molecules are attached to it. Hemoglobin is the protein that carries oxygen. Since red blood cells live for about two to three months, the A1C percentage provides a weighted average of blood glucose levels over that entire period. This long-term measure is valuable for assessing overall glycemic control and evaluating treatment effectiveness.
The Estimated Average Glucose (eAG) Calculation
The established medical method for bridging the gap between the A1C percentage and the daily mg/dL reading is the Estimated Average Glucose, or eAG. The eAG is a calculated value designed to express the A1C result in the familiar units of a standard blood glucose meter. This translation makes the long-term A1C result more intuitive for individuals managing their blood sugar.
The relationship between A1C and eAG was established by the A1C-Derived Average Glucose (ADAG) study. This research correlated A1C results with thousands of frequent blood glucose measurements over three months, leading to a specific mathematical formula for conversion. The formula for calculating eAG in mg/dL from the A1C percentage is: eAG = 28.7 × A1C – 46.7.
The resulting eAG value is an estimate, not a direct, real-time measurement of glucose concentration. It is an average that accounts for all hours of the day and night. This calculated average may not perfectly match the average of an individual’s self-monitored readings, which are often taken before meals or at other specific times. The eAG serves as a standardized way to help patients understand their long-term blood glucose control.
Interpreting A1C Results and Target Levels
The conversion to eAG provides a practical context for interpreting the medical significance of the A1C result. An A1C of 6%, for example, corresponds to an eAG of 126 mg/dL, meaning the average blood glucose level over the preceding months was approximately 126 mg/dL. A 7% A1C is equivalent to an eAG of 154 mg/dL, and an A1C of 8% translates to an eAG of 183 mg/dL.
Healthcare organizations use these values to define diagnostic and management goals. An A1C below 5.7% is typically considered a normal range, while an A1C between 5.7% and 6.4% suggests prediabetes. A diagnosis of diabetes is indicated by an A1C of 6.5% or higher.
For most non-pregnant adults with diabetes, major health organizations recommend a target A1C of less than 7%. This target corresponds to an eAG of less than 154 mg/dL. The specific goal is individualized, however, as factors like age, other health conditions, and risk of low blood sugar episodes may lead a healthcare provider to set a slightly higher or lower target. Understanding the eAG conversion allows individuals to correlate their long-term A1C percentage with the daily readings they see on their glucose meter.