A1C vs. Average Glucose: What Your Lab Results Really Mean for Diabetes Management

When you get your diabetes lab results back, you might see two numbers that seem to tell the same story: your A1C and your average glucose. One is a percentage, the other is in mg/dL. At first glance, they look different. But they’re both trying to answer the same question: How well is your blood sugar being managed over time? The problem? Many people think they’re interchangeable. They’re not. And misunderstanding the difference can lead to missed risks, poor decisions, and even dangerous blind spots in your care.

What A1C Actually Measures

A1C-also called HbA1c-is a blood test that shows your average blood sugar over the past two to three months. It works by measuring how much glucose has stuck to the hemoglobin in your red blood cells. Red blood cells live for about 120 days, so the test gives you a long-term snapshot. It doesn’t care if you had a spike after dinner last Tuesday or dropped low at 3 a.m. Wednesday. It only sees the big picture.

The American Diabetes Association (ADA) uses A1C ≥6.5% as the official diagnostic cutoff for diabetes. For most adults with diabetes, the target is below 7.0%. But that’s not one-size-fits-all. If you’re older, have heart disease, or get frequent low blood sugars, your doctor might aim for 7.5% or even 8.0%. If you’re younger and healthy, they might push for 6.5% or lower. It’s personal.

What makes A1C so popular? It’s stable. You don’t need to fast. It’s not affected by what you ate that morning or how stressed you were. It’s a reliable, lab-confirmed number that doesn’t change day to day. That’s why doctors rely on it. But here’s the catch: it hides the details.

What Average Glucose Tells You That A1C Doesn’t

When you check your blood sugar with a meter or a continuous glucose monitor (CGM), you’re seeing real-time numbers: 110, 185, 68, 142. These are snapshots. But if you average them over days or weeks, you get something called estimated average glucose (eAG) or Glucose Management Indicator (GMI).

eAG is just A1C converted into mg/dL using a simple formula: 28.7 × A1C - 46.7. So if your A1C is 7.0%, your eAG is 154 mg/dL. That means your average blood sugar over the last few months was around 154 mg/dL. Suddenly, that percentage feels more real. You can picture it.

GMI works the same way-but it’s calculated from your CGM data. If you’ve worn a CGM for 14 days or more, your device can give you a GMI number. It’s more accurate than eAG because it uses hundreds or thousands of readings, not just a lab test.

Here’s why this matters: A1C can be normal while your daily glucose is all over the place. You could spend 12 hours a day under 70 mg/dL and 10 hours above 200 mg/dL, and still have an A1C of 6.8%. Your doctor sees “good control.” But you’re living with dangerous swings. That’s not control-that’s chaos.

The Hidden Danger: A1C Can Lie

There are real situations where A1C doesn’t match your true blood sugar pattern. People with anemia, kidney disease, liver disease, or certain blood disorders can have falsely low or high A1C results. Iron deficiency? That can make your A1C look better than it is. Sickle cell trait? That can throw off the test entirely.

Even if your blood is normal, A1C still misses the spikes and drops that cause damage. Research from the Journal of Diabetes Science and Technology shows that glucose variability-those big swings-is a stronger predictor of complications than average levels alone. A1C doesn’t see them. CGM does.

One patient on a diabetes forum said: “My A1C was 6.8%. My doctor said I was doing great. But my CGM showed I was below 70 mg/dL 8% of the time. I passed out twice last month. No one asked about that.” That’s not rare. A 2021 survey found that 31% of people with diabetes had hypoglycemia episodes ignored because their A1C was “in range.”

How to Use Both Numbers Together

The best approach isn’t choosing between A1C and average glucose. It’s using them together.

• Use A1C as your long-term benchmark. Test it every 3-6 months.
• Use eAG or GMI to understand what that percentage feels like in daily life.
• Use CGM data to see your Time in Range (TIR): how many hours you spend between 70-180 mg/dL. Aim for at least 70%.
• Watch your Time Below 70 mg/dL. Keep it under 4%.
• Watch your Time Above 180 mg/dL. Keep it under 25%.

For example: If your A1C is 7.2% (eAG = 161 mg/dL), but your CGM shows you’re only in range 55% of the time and dropping below 70 mg/dL 6% of the time, your treatment plan needs adjusting. Maybe you’re overcorrecting with insulin at night. Maybe you’re eating too many carbs at dinner. A1C alone won’t tell you that.

Doctors are starting to catch on. The ADA’s 2024 guidelines now recommend using TIR and GMI alongside A1C, especially for people using CGMs. In fact, some clinical trials now use TIR as the main outcome instead of A1C. By 2027, experts predict TIR will become the primary target-and A1C will be the backup.

What Your Targets Should Be

There’s no universal “perfect” number. Your goal depends on your age, health, lifestyle, and risk of low blood sugar.

These aren’t just numbers. They’re safety zones. Spending too much time above 180 mg/dL slowly damages your nerves, kidneys, and eyes. Spending too much time below 70 mg/dL can cause seizures, falls, or even death.

Real-World Examples

Let’s say you’re 58, have type 2 diabetes, and your A1C is 6.9%. You feel fine. Your doctor says you’re doing great.

But your CGM tells a different story. You’re in range only 58% of the time. You’re below 70 mg/dL for 7% of the day-mostly overnight. You’ve had three unexplained falls in the last three months. Your A1C looks good, but your daily reality is risky.

Now compare that to a 32-year-old with type 1 diabetes. Her A1C is 7.1%, but her CGM shows she’s in range 82% of the time and never goes below 70 mg/dL. Her swings are small. She’s stable. Her A1C is slightly higher, but her health is better.

A1C alone can’t tell you which of these two people is truly doing better. You need the full picture.

What to Do Next

Don’t wait for your next A1C test to understand your control. Here’s what to do right now:

1. If you’re not using a CGM, ask your doctor about one. Even a 14-day trial can show you patterns you’ve never seen.
2. Check your CGM reports for Time in Range, Time Below, and Time Above. Don’t just look at the average.
3. Calculate your GMI. Most CGM apps do it automatically. Compare it to your A1C. If they’re more than 0.5% apart, talk to your provider.
4. Write down any lows or highs you notice-even if you think they’re “normal.”
5. Bring your CGM data and your A1C to your next appointment. Say: “I want to understand how these numbers fit together.”

Diabetes isn’t about hitting one magic number. It’s about minimizing the swings, staying safe, and living well. A1C is a tool. Average glucose is a tool. But your daily experience? That’s the real data.