Common Mistakes Interpreting A1C and eAG Results

July 14, 2026

A1C and eAG are two of the most useful numbers in diabetes care, but they’re also two of the most commonly misread. Here are the mistakes that show up most often — and how to avoid them.

Mistake 1 — Treating A1C as a Daily Blood Sugar Reading

A1C is a two-to-three-month average, not a snapshot. It cannot tell you what your blood sugar was this morning, and it won’t move noticeably after one good or bad week. People sometimes expect a diet change from the past few days to show up on their next A1C — it won’t, because the test reflects the average glucose exposure of red blood cells over their roughly three-month lifespan. Use daily glucose meter or CGM readings to track day-to-day patterns, and A1C to track the longer-term trend.

Mistake 2 — Assuming Identical A1C Means Identical Blood Sugar Control

Two people can share the same A1C — say, 7.0% — while having very different glucose patterns. One might hover consistently around 154 mg/dL. The other might swing widely between 80 mg/dL lows and 250 mg/dL highs that average out to the same eAG. A1C alone can’t distinguish stable control from a rollercoaster pattern; that’s what CGM data and time-in-range metrics are for. Don’t assume a “good” A1C automatically means minimal glucose variability.

Mistake 3 — Confusing mg/dL and mmol/L

The US reports glucose in mg/dL; most other countries use mmol/L. These are different scales — 154 mg/dL and 154 mmol/L are wildly different numbers (the second would be dangerously, impossibly high). If you’re reading results from an international lab, a device set to the wrong region, or comparing notes with someone outside the US, double-check which unit is in use before assuming a number is high or low. Run the conversion through the A1C calculator rather than estimating — it converts A1C, mg/dL, mmol/L, and IFCC mmol/mol simultaneously so there’s no unit guesswork. See the A1C, eAG & ADAG formula glossary for exactly how each unit relates to the others.

Mistake 4 — Ignoring Conditions That Can Skew A1C

A1C depends on red blood cells living their normal ~3-month lifespan and picking up glucose along the way. Certain anemias, hemoglobin variants (like sickle cell trait), recent blood transfusions, pregnancy, and chronic kidney or liver disease can all shorten or lengthen that lifespan — which shifts the A1C percentage independent of actual average glucose. Someone with one of these conditions who reads their A1C at face value could be misled in either direction. If your A1C and your home glucose readings seem inconsistent, one of these factors — not measurement error — may be why. See ADA A1C diagnostic ranges explained for the full list of conditions that can affect accuracy.

Mistake 5 — Diagnosing from a Single Test

A1C in the diabetes range (6.5% or higher) on one test is not automatically a final diagnosis. Standard practice calls for a repeat test on a separate day, unless clear symptoms and a random glucose of 200 mg/dL or higher are already present. Self-diagnosing — or assuming a single borderline result is definitive — skips a step that exists specifically to catch lab variability and one-off errors.

Mistake 6 — Reading the Reverse (Glucose-to-A1C) Formula as Exact

It’s possible to estimate what a lab A1C might read using average glucose from a CGM or meter, with the formula A1C (%) = (eAG in mg/dL + 46.7) ÷ 28.7. This is a helpful estimate — but it’s not a substitute for a lab-drawn A1C. Device accuracy, calibration, and the length of the averaging window all introduce differences between a CGM-estimated A1C and the actual lab value.

Mistake 7 — Panicking Over a Small Month-to-Month Change

Small A1C fluctuations (say, 6.6% to 6.8%) between two tests a few months apart are common and don’t necessarily reflect a meaningful change in health status — lab variability alone accounts for some of it. Trends over multiple tests matter more than any single fluctuation.

For worked examples that show exactly how specific A1C values convert to eAG and IFCC, see A1C to average glucose conversion examples. If you’re working through a first-time result, the guide for newly diagnosed prediabetes and diabetes covers what to expect next.