If you use a continuous glucose monitor (CGM), you've probably noticed a number called GMI, or Glucose Management Indicator. It often looks similar to an A1C result. For example: GMI 6.8%, laboratory A1C 7.2%.
Many people immediately wonder: "Which one is correct?"
The answer is that both numbers provide useful information, but they are not measuring exactly the same thing. Understanding the difference can help you interpret your diabetes data more confidently and have more productive conversations with your healthcare provider.
What Is an A1C?
An A1C is a laboratory test performed on a blood sample. It estimates your average blood sugar over approximately the previous two to three months by measuring how much glucose has attached to hemoglobin inside your red blood cells. Because it is a laboratory measurement, the A1C has been used for decades to diagnose diabetes and monitor long-term glucose control.
What Is GMI?
GMI stands for Glucose Management Indicator. Unlike the A1C, GMI is not a laboratory test. Instead, it is a mathematical estimate generated from your CGM data.
Your CGM calculates your average glucose over the period you've been wearing the sensor and then estimates what your A1C might be if those glucose levels continued over time. In other words: A1C is measured. GMI is estimated.
How Is GMI Calculated?
Every CGM collects hundreds of glucose readings each day. Using those readings, the device calculates your average glucose, which is then converted into a GMI using a standardized formula developed from clinical research. You don't need to perform any calculations yourself—your CGM app usually displays GMI automatically once enough data have been collected.
Why Don't GMI and A1C Always Match?
One of the most common misconceptions is that GMI and A1C should always be identical. They often aren't. Several factors explain why.
They measure different things
The A1C measures glucose attached to hemoglobin inside red blood cells. GMI estimates an A1C based on glucose readings collected by your CGM. Although they are related, they are fundamentally different measurements.
They cover different time periods
An A1C reflects roughly the previous two to three months of glucose control. A GMI reflects only the time your CGM has collected data, often the previous 10 to 14 days or several weeks, depending on the amount of available data. If your diabetes management has recently improved—or worsened—your GMI may change much sooner than your laboratory A1C.
Red blood cells differ between people
The A1C depends on the lifespan of your red blood cells. Some medical conditions can shorten or lengthen red blood cell survival, affecting the A1C even when glucose levels remain the same. GMI is not influenced by red blood cell lifespan because it comes directly from CGM readings.
Your CGM doesn't capture every moment
Although modern CGMs collect an enormous amount of information, occasional gaps may occur if the sensor is removed, signal is temporarily lost, the sensor fails, or the device isn't worn consistently. Less complete data can make the GMI less representative of your overall glucose patterns.
Which Number Should You Trust?
Neither measurement is "better." They answer different questions. The A1C tells you how your glucose has averaged over the past several months. GMI tells you what your recent CGM data suggest about your overall glucose control. Rather than choosing one over the other, your healthcare provider looks at both numbers together.
What If My GMI Is Lower Than My A1C?
This is common. It may mean your glucose control has improved recently, your recent CGM data are better than your earlier glucose patterns, or the normal differences between GMI and A1C are simply becoming apparent. Your next laboratory A1C may move closer to your GMI if those improvements continue.
What If My GMI Is Higher Than My A1C?
This may occur if your blood sugar has recently become more difficult to control, you have experienced recent illness or stress, your medications have changed, or your recent CGM data represent a period of higher glucose than the previous several months. Looking at longer-term trends often helps explain these differences.
Don't Focus on One Number
Whether you're reviewing GMI or A1C, avoid becoming discouraged by a single result. Instead, consider the complete picture: Time in Range, average glucose, frequency of hypoglycemia, glucose variability, daily glucose patterns, your symptoms, and your overall health.
Good diabetes management is about understanding trends—not chasing individual numbers.
How a CGM Adds Valuable Information
An A1C tells you how high your glucose has been on average. A CGM tells you when glucose rises, when glucose falls, how often you experience hypoglycemia, how meals affect you, how exercise affects you, and whether your glucose is stable or highly variable. This additional information helps you and your healthcare provider make more informed treatment decisions.
Frequently Asked Questions
Is GMI the same as A1C?
No. GMI is an estimate based on CGM data. A1C is a laboratory measurement based on glucose attached to red blood cells.
Why is my GMI lower than my A1C?
Recent improvements in blood sugar are one common reason. Because GMI reflects more recent glucose data, it often responds more quickly than the laboratory A1C.
Can my doctor use GMI instead of an A1C?
Not usually. The A1C remains an important laboratory test for monitoring diabetes. GMI is a valuable additional tool but does not replace recommended laboratory testing.
Which number matters more?
Both matter. Together with Time in Range, glucose patterns, and your overall health, they provide a more complete picture of diabetes management than either measurement alone.
When Should You Contact Your Healthcare Provider?
Contact your healthcare provider if your GMI and A1C differ significantly and you don't understand why, your blood sugar has recently changed dramatically, your CGM data don't seem to match your symptoms, you have questions about interpreting your diabetes reports, or your glucose is consistently above or below your target range. Your healthcare provider can review your CGM reports and laboratory results together to determine whether changes to your treatment plan are needed.
Seek Immediate Medical Care If
Seek emergency medical attention if you experience severe hypoglycemia with confusion, seizure, or loss of consciousness; persistent very high blood sugar with vomiting or difficulty breathing; symptoms of diabetic ketoacidosis (abdominal pain, rapid breathing, or confusion); or any other signs of a diabetes-related emergency. Neither GMI nor A1C should delay evaluation of urgent symptoms.
The Bottom Line
GMI and A1C are both valuable tools for understanding diabetes, but they measure different aspects of glucose control. Your A1C is a laboratory test that reflects average blood sugar over the previous two to three months. GMI is an estimate generated from your recent CGM data. Because they measure different things and cover different time periods, they won't always match exactly.
Rather than worrying about small differences between the two, focus on the overall picture—including your Time in Range, glucose trends, frequency of highs and lows, and how you feel. Reviewing all of these together with your healthcare provider provides the most complete understanding of your diabetes management.