When you get blood work for diabetes screening or management, your results may include two numbers: an HbA1c percentage and an eAG estimation. They measure the same thing in two different ways. HbA1c (hemoglobin A1c) tells you what percentage of your red blood cells have glucose attached to them, reflecting your average blood sugar over the past three months. The eAG (estimated average glucose) converts that percentage into the same units you see on a home glucose meter or continuous glucose monitor, typically mg/dL.
How HbA1c Works
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. The more glucose circulating over time, the more hemoglobin gets coated. Since red blood cells live about 90 to 120 days, measuring the percentage of hemoglobin with glucose attached gives a reliable picture of your average blood sugar over roughly three months. A result of 6%, for example, means 6% of your hemoglobin has been glycated.
HbA1c has been the standard marker for blood sugar control since the mid-1970s and remains the best predictor of diabetes-related complications affecting the eyes, kidneys, and nerves. It’s used both to diagnose diabetes (a result of 6.5% or higher) and to track how well treatment is working over time.
What eAG Adds
The problem with a percentage is that it doesn’t match anything you see day to day. If your meter reads 154 mg/dL one morning, you have no intuitive sense of how that relates to an A1c of 7%. That disconnect is exactly what eAG solves. It translates the A1c percentage into the same mg/dL (or mmol/L) units your meter uses, so you can compare the two more easily.
The American Diabetes Association and the American Association for Clinical Chemistry determined that the correlation between A1c and average glucose is strong enough (r = 0.92) to justify reporting both numbers together whenever a clinician orders an A1c test. So when your lab report lists “HbA1c with eAG estimation,” it simply means the lab ran one test (the A1c) and then calculated the corresponding average glucose for you.
The Conversion Formula
The eAG calculation comes from a landmark study called the A1c-Derived Average Glucose (ADAG) trial. Researchers tracked hundreds of people using continuous glucose monitors and frequent fingerstick readings, then mapped those real-world averages against their A1c results. The formula they established is:
eAG (mg/dL) = 28.7 × A1c – 46.7
Here’s what that looks like at common A1c levels:
- A1c 5.7%: eAG of about 117 mg/dL (the threshold for prediabetes)
- A1c 6%: eAG of 126 mg/dL (7.0 mmol/L)
- A1c 6.5%: eAG of 140 mg/dL (7.8 mmol/L)
- A1c 7%: eAG of 154 mg/dL (8.6 mmol/L)
- A1c 8%: eAG of 183 mg/dL (10.1 mmol/L)
- A1c 9%: eAG of 212 mg/dL (11.8 mmol/L)
- A1c 10%: eAG of 240 mg/dL (13.4 mmol/L)
For most adults with diabetes, the treatment target is an A1c below 7%, which corresponds to an eAG under 154 mg/dL.
Why eAG May Not Match Your Meter Average
One common source of confusion: the eAG on your lab report probably won’t match the average glucose displayed on your meter or CGM app. That’s normal, and it doesn’t mean either number is wrong.
Your meter average only reflects the moments you actually checked. Most people test more often when blood sugar is likely to be lower, such as first thing in the morning or right before meals, and less often after meals when glucose spikes. That skews your meter average downward. The eAG, by contrast, represents your glucose levels around the clock, including the post-meal highs you may rarely catch with fingersticks. If you use a CGM that samples every few minutes, the gap between your CGM average and eAG will typically be much smaller.
When the Numbers Can Be Misleading
Because A1c depends on hemoglobin inside red blood cells, anything that changes your hemoglobin or shortens the lifespan of your red blood cells can throw off the result, and since eAG is calculated directly from A1c, both numbers shift together.
Conditions that can cause a falsely low A1c include sickle cell disease, other hemoglobin variants (like hemoglobin C), thalassemia, and certain anemias where red blood cells are destroyed faster than normal. When red blood cells don’t survive as long, there’s less time for glucose to accumulate on them, making blood sugar appear better controlled than it actually is. Heavy or recent blood loss and blood transfusions can have similar effects.
Falsely high readings are less common but can occur with iron deficiency anemia (where red blood cells live longer than usual) or in people with elevated levels of fetal hemoglobin, which can result from conditions like leukemia or a hereditary trait. Pregnancy also affects red blood cell turnover in ways that can make A1c unreliable, particularly in the second and third trimesters.
In any of these situations, your provider may rely on alternative markers of blood sugar control, such as fructosamine or glycated albumin, which reflect shorter time windows and aren’t affected by red blood cell lifespan.
How to Use Both Numbers
Think of A1c as the big-picture score and eAG as the translation into everyday language. If your A1c is 7.5% and your eAG is 169 mg/dL, that tells you your blood sugar has been averaging around 169 over the past three months. You can then compare that to the readings you see on your meter and ask: where are the gaps? If your fasting numbers look fine but your eAG is higher than expected, post-meal spikes or overnight highs you aren’t catching could be the culprit.
Tracking A1c and eAG over multiple lab visits also shows trends. A drop from an eAG of 183 to 154 mg/dL means your A1c went from 8% to 7%, a meaningful improvement that lowers the risk of long-term complications. Seeing that change in glucose units rather than abstract percentages makes the progress feel more concrete and easier to connect to daily choices.