Q: What is estimated average glucose (eAG)?

eAG expresses HbA1c in the same units as a glucose meter (mg/dL or mmol/L), making the percentage easier to relate to everyday readings. It comes from the ADAG study regression: eAG (mg/dL) = 28.7 × HbA1c − 46.7, with eAG (mmol/L) = mg/dL ÷ 18. For example, an HbA1c of 7.0% corresponds to an eAG of about 154 mg/dL (8.6 mmol/L). eAG is a translation of the average, not a measured glucose, so an individual's true day-to-day values still vary around it.

Q: How often should HbA1c be measured?

For people with stable, at-goal diabetes, HbA1c is typically checked about every 6 months; when therapy has changed or glycemic goals are not being met, roughly every 3 months is common, since that interval matches how long the marker takes to reflect a new steady state. Because HbA1c averages 2–3 months of glucose, checking it more frequently than quarterly rarely adds information.

Question 1

What is the difference between HbA1c and a daily fingerstick glucose?

Accepted Answer

A fingerstick (or CGM) reading is the glucose right now and swings with meals, activity, and stress. HbA1c is a longer-term average: it measures the fraction of hemoglobin that has become glycated, which reflects mean glucose over the prior 2–3 months. The two are linked through estimated average glucose (eAG) — this tool converts an HbA1c percentage into the average glucose it corresponds to — but HbA1c cannot show day-to-day highs and lows or hypoglycemia, which is why it complements rather than replaces glucose monitoring.

Question 2

Why does HbA1c reflect the past 2–3 months?

Accepted Answer

Glucose attaches to hemoglobin inside red blood cells slowly and irreversibly, so the proportion that is glycated builds up over the cell's lifespan of roughly 120 days. HbA1c therefore represents a weighted average of blood glucose over the preceding 8–12 weeks, with the most recent month contributing more than earlier weeks. That long window is what makes it useful for tracking overall control, and also why a change in therapy takes a couple of months to show up fully in the number.

Question 3

When is HbA1c unreliable?

Accepted Answer

Anything that changes red-blood-cell turnover or interferes with the assay can distort it. Shortened cell survival — hemolytic anemia, recent blood loss or transfusion, pregnancy, erythropoietin therapy — tends to lower HbA1c falsely, while conditions that prolong cell survival, such as iron-deficiency anemia and asplenia, tend to raise it. Hemoglobin variants and chronic kidney disease can shift it in either direction depending on the method. In these situations HbA1c should be interpreted with caution and direct glucose measures (fructosamine, CGM, or plasma glucose) may be more reliable.

Question 4

Why is the control target often 7%?

Accepted Answer

The ADA's general target for many non-pregnant adults with diabetes is below 7.0% (about eAG 154 mg/dL) because large trials found that lowering HbA1c to roughly this level reduces long-term microvascular complications while keeping hypoglycemia risk acceptable. It is a starting reference, not a universal rule: a more stringent target (e.g., < 6.5%) may suit someone young with low hypoglycemia risk, while a looser one (e.g., < 8.0%) fits limited life expectancy, frailty, or significant comorbidity. The right individual target is set by the treating clinician; this page only interprets where a value falls.

Question 5

What is estimated average glucose (eAG)?

Accepted Answer

eAG expresses HbA1c in the same units as a glucose meter (mg/dL or mmol/L), making the percentage easier to relate to everyday readings. It comes from the ADAG study regression: eAG (mg/dL) = 28.7 × HbA1c − 46.7, with eAG (mmol/L) = mg/dL ÷ 18. For example, an HbA1c of 7.0% corresponds to an eAG of about 154 mg/dL (8.6 mmol/L). eAG is a translation of the average, not a measured glucose, so an individual's true day-to-day values still vary around it.

Question 6

How often should HbA1c be measured?

Accepted Answer

For people with stable, at-goal diabetes, HbA1c is typically checked about every 6 months; when therapy has changed or glycemic goals are not being met, roughly every 3 months is common, since that interval matches how long the marker takes to reflect a new steady state. Because HbA1c averages 2–3 months of glucose, checking it more frequently than quarterly rarely adds information.

HbA1c (%)	Classification	eAG (mg/dL)	eAG (mmol/L)
5.0	Normal	97	5.4
5.7	Prediabetes (lower bound)	117	6.5
6.0	Prediabetes	126	7.0
6.5	Diabetes (diagnostic)	140	7.8
7.0	Diabetes — common adult target	154	8.6
8.0	Diabetes — above target	183	10.2
9.0	Diabetes — above target	212	11.8
10.0	Diabetes — above target	240	13.3

HbA1c Calculator (A1c to eAG)

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