Why Your A1C Number Doesnât Tell the Whole Story
You get your A1C result back: 7.1%. Your doctor says youâre doing well. But your glucose meter shows spikes over 200 mg/dL after meals and drops below 60 mg/dL at night. Youâre confused. Why does your A1C look good when your daily numbers feel chaotic?
This is more common than you think. A1C is a powerful tool-but itâs not a complete picture. Itâs like looking at a seasonâs batting average in baseball. If you hit .300, youâre a solid hitter. But that doesnât tell you if you struck out in the 9th inning of Game 7, or if you got a home run every other game. A1C averages your blood sugar over 2-3 months. It smooths out the highs and lows. And thatâs exactly where the problem starts.
What A1C Actually Measures
A1C, or HbA1c, is a lab test that measures how much glucose has stuck to your red blood cells. Red blood cells live for about 120 days. As they circulate, glucose in your blood attaches to them. The more glucose you have, the more sticks. Thatâs what A1C captures: the average amount of glucose your blood cells have been exposed to over the past few months.
Itâs not affected by what you ate yesterday or whether you skipped breakfast. Thatâs why itâs so useful. No fasting. No stress. Just a simple blood draw. The American Diabetes Association (ADA) uses A1C â„6.5% as the official cutoff for diagnosing diabetes. For most adults with diabetes, the target is under 7.0%. But that target isnât one-size-fits-all. For older adults or those with heart disease, 7.5% might be safer. For younger people with type 1 diabetes, aiming for 6.5% or lower is often recommended.
Hereâs the math behind it. The ADA uses this formula to convert A1C to estimated average glucose (eAG): 28.7 Ă A1C - 46.7 = eAG. So:
- A1C 6.0% = 126 mg/dL
- A1C 7.0% = 154 mg/dL
- A1C 8.0% = 183 mg/dL
- A1C 9.0% = 212 mg/dL
Thatâs helpful. But hereâs the catch: this formula was built from data collected from people who checked their blood sugar 7-8 times a day for 3 months. Most people donât do that. So the real average glucose you see on your meter might not match your A1C.
What Average Glucose Really Means
Average glucose is what you see on your glucose meter or continuous glucose monitor (CGM). If you check your blood sugar 4 times a day for a week, you can calculate the average. But thatâs just a snapshot. A CGM gives you 288 readings a day. Thatâs way more accurate.
Thatâs where GMI comes in-Glucose Management Indicator. Itâs the CGM version of eAG. Instead of using a formula based on A1C, GMI calculates your average glucose directly from your CGM data using the same math: GMI (mg/dL) = 12.71 + 4.70587 Ă mean glucose. The result? A number that matches your daily experience.
Hereâs the big difference: A1C is a backward-looking lab test. GMI is a real-time, data-rich summary from your monitor. If your CGM shows youâre spending 8% of your day below 70 mg/dL, your GMI will reflect that. Your A1C? It might still say 6.8%. Thatâs because the lows are averaged out with the highs.
When A1C Lies to You
A1C can be misleading. It doesnât show you how wild your numbers swing. You could have perfect A1C but spend half your day in hypoglycemia and the other half in hyperglycemia. Thatâs called glycemic variability-and itâs dangerous.
A 2021 study of over 5,700 people found that many with A1C under 7% had frequent low blood sugars. One user on Diabetes Daily wrote: âMy A1C was 6.8%, but my CGM showed I was below 70 mg/dL for 8% of the time. My doctor didnât see it because he only looked at the A1C.â Thatâs not rare. In fact, a 2021 survey by Diabetes Sisters found that 31% of people said their providers missed hypoglycemia because their A1C was âin range.â
And itâs not just lows. Some people have high post-meal spikes that donât show up in A1C because theyâre short-lived. One user reported: âMy meter average was 140 mg/dL, but my A1C was 7.2%. The eAG calculator said 154 mg/dL. Turns out, I was hitting 220 mg/dL after dinner every night.â Thatâs the hidden cost of averaging.
Why Both Numbers Matter Together
The best approach isnât choosing between A1C and average glucose-itâs using both.
Think of A1C as your annual report card. It tells you if youâre generally on track. GMI or your CGM average is your weekly progress report. It shows whatâs happening right now.
Doctors are starting to use both. The American Association of Clinical Endocrinologists now recommends looking at:
- Time in Range (70-180 mg/dL): Aim for at least 70% of the day
- Time below 70 mg/dL: Keep it under 4%
- Time above 180 mg/dL: Keep it under 25%
These numbers come from CGM data. A1C still matters-but now itâs a backup. If your GMI says 160 mg/dL and your A1C is 7.5%, somethingâs off. Maybe your red blood cells are living longer due to iron deficiency. Or maybe youâve had recent blood loss. A1C can be wrong in people with anemia, kidney disease, or certain blood disorders.
A 2023 study from the T1D Exchange registry found that using both A1C and GMI reduced treatment errors by 23%. Thatâs huge. It means fewer missed lows, fewer unnecessary insulin changes, and better outcomes.
What You Should Do Right Now
If youâre not using a CGM, start tracking your glucose 4 times a day for at least 3 weeks. Write down your numbers: before meals, after meals, at bedtime, and if you feel low. Calculate your average. Then compare it to your last A1C.
If your average glucose is much higher than your eAG (calculated from A1C), youâre likely having big spikes. If itâs lower, you might be having lows that your A1C hides.
If you have a CGM, look at your 14-day report. Donât just glance at the average. Look at the graph. How much time are you spending below 70? Above 180? Are your highs happening after dinner? Are your lows happening at 3 a.m.?
Take that report to your doctor. Say: âI want to understand how my daily numbers connect to my A1C.â Donât let them just say, âYouâre good.â Ask: âIs my Time in Range where it should be? Are my lows being ignored?â
What the Future Holds
By 2027, experts predict Time in Range will replace A1C as the main target for diabetes care. A1C wonât disappear-itâll become a secondary check. The FDA has already approved clinical trials to use TIR as the primary outcome. Thatâs a major shift.
Why? Because we now know that glucose variability matters as much as average levels. Fluctuations damage blood vessels. They increase heart disease risk. A1C canât see that. CGM data can.
For now, youâre stuck with both systems. But you donât have to be confused. Use A1C to see the big picture. Use your average glucose to fix the daily problems. One tells you where youâve been. The other tells you where you need to go.
Quick Reference: A1C to eAG Conversion
| A1C (%) | eAG (mg/dL) |
|---|---|
| 5.0 | 97 |
| 6.0 | 126 |
| 7.0 | 154 |
| 8.0 | 183 |
| 9.0 | 212 |
| 10.0 | 240 |
| 11.0 | 269 |
| 12.0 | 298 |
What to Ask Your Doctor
- Whatâs my current A1C, and whatâs my estimated average glucose (eAG)?
- Do I have a CGM report? Can we look at my Time in Range and time below 70 mg/dL?
- Is my A1C consistent with my daily glucose readings? If not, why?
- Based on my lifestyle and health, whatâs my personal A1C target?
- Are my lows being tracked, or is my A1C hiding them?
Saket Modi
December 3, 2025 AT 07:50