Using GMI To Estimate Your A1C: How Accurate Is It?
Key takeaways
- While an A1C blood test has long been the standard for assessing blood sugar management, it can often be inaccurate in people with diabetes.
- GMI is a metric that uses CGM data to estimate your A1C. Using newer metrics like GMI and time in range can help personalize diabetes care and improve management.
- Compared to A1C or GMI, time in range provides more information on glucose variability than measures based on averages.
An A1C test is commonly used to assess a person’s glucose management. It’s a type of blood test that measures your average blood sugar levels over two to three months. A higher A1C is associated with higher blood glucose levels and a higher risk for health complications. Although long considered a gold standard in diabetes care, A1C has a huge limitation: It’s just an average.
A person could spend a lot of time with low blood sugar levels and a lot of time with high levels, yet have an A1C under 7%, which is the target for most people with diabetes. Part of the limitation of A1C is that it doesn’t capture day-to-day fluctuations in blood sugar. Additionally, in people with anemia, chronic kidney disease, or other hemoglobin disorders, A1C may be less accurate as these conditions can affect the lifespan of red blood cells.
For those who use continuous glucose monitoring (CGM), the glucose management indicator (GMI) is a metric that essentially estimates your A1C. Without the need for a blood draw, it takes the average of your glucose readings from your CGM and uses a formula to estimate what your A1C is expected to be. To calculate GMI, you should have at least 14 days of data.
Your blood sugar management can also be measured by time in range (time spent between 70-180 mg/dL), time below range (below 70 mg/dL), and time above range (over 180 mg/dL). Generally, a time in range of 70% or greater is equivalent to an A1C of 7% or less. This data is quite useful to see if you are meeting your glucose targets or if you need to adjust your diabetes treatment plan.
Differences between GMI and A1C
An A1C is based on red blood cell turnover. It provides your average blood glucose levels over two to three months, since that’s how long it takes for new red blood cells to form. For this reason, an A1C blood test is usually only obtained quarterly. GMI, on the other hand, does not require a blood draw and can be checked much more frequently since it is based on CGM data.
But how accurate is GMI? Is it realistic to expect that your A1C will be exactly what the GMI shows on your CGM report?
It’s important to keep in mind that GMI and A1C are different. Since A1C is based on red blood cells, anything that affects them could falsely skew your A1C number. This includes anemia and genetic conditions that affect hemoglobin (a protein found in red blood cells), such as sickle cell disease. Many people with chronic kidney disease also have falsely low A1C. Studies show that people of color, including Black, Asian, Hispanic, and Indigenous individuals, have slightly higher average A1C levels compared to white people.
Comparing GMI and A1C, the numbers are usually close, but aren’t likely to be an exact match. It’s normal for your GMI to be higher or lower than your A1C.
What does the research say?
Despite GMI’s widespread use in diabetes care, little real-world data exists on how accurately it reflects A1C levels.
In one study comparing GMI to A1C, researchers found that 22% of participants had differences of 1 percentage point or more between their GMI and A1C. Researchers found slightly smaller differences in people with type 1 diabetes compared to those with type 2 diabetes, and greater differences in those with reduced kidney function – though this was less surprising, as kidney disease can falsely lower A1C values. The CGM device type did not significantly affect the results.
More recent studies have found that even in individuals with type 2 diabetes who had an A1C below 7% and time in range above 70%, there was still a discrepancy between their A1C and GMI. Researchers have identified several factors that may contribute to discrepancies between GMI and A1C, including age, diabetes duration, and variations in red blood cell size – all of which can significantly influence the difference between GMI and A1C values.
So while the GMI provides a clue as to what your next A1C result might be, don’t be surprised if they aren’t a perfect match. The GMI may actually be more reflective of your true average glucose levels since there is no lab interference. Many healthcare offices relied on GMI more during COVID-19 when it was challenging to get A1C lab tests in person.
Pros and cons of GMI
Studies have highlighted both the benefits and limitations of using GMI in diabetes care. On the one hand, researchers believe that a key advantage of GMI is that it can be calculated directly from CGM data, offering an accessible estimate of A1C without the need for lab testing. This may reduce costs and allow for remote diabetes monitoring, eliminating the need for in-person visits.
However, other experts have challenged the ongoing use of GMI, stating that it’s a poor estimate of A1C and that using mean glucose from CGM data is a more accurate and straightforward alternative.
Other experts acknowledged the value of using mean glucose, but noted that most clinicians lack a frame of reference for interpreting mean glucose values at this time. In contrast, A1C has been widely used for decades, and its clinical significance is well understood. Because of this, many believe that GMI still serves an important role in helping people with diabetes and their providers interpret CGM data and bridge the gap toward more individualized diabetes management.
Additional research has raised concerns about the reliability of GMI. For example, a review of type 1 diabetes clinical trials found that GMI may not be a dependable endpoint for evaluating treatment efficacy. Another study in individuals without diabetes showed that GMI often fails to accurately estimate A1C.
As a result, many people with diabetes and healthcare professionals are increasingly turning to CGM metrics like time in range, which offer more timely, detailed, and actionable insights than A1C or GMI alone.
Focusing on time in range allows you to compare blood sugar management strategies on a day-to-day basis. If you can increase your time in range, your GMI and A1C are much more likely to also reach their targets.
The bottom line
GMI is a metric that provides an estimated A1C based on your CGM data. Unlike an A1C test, GMI doesn’t require a blood draw. Though GMI can provide an estimate, it’s not a perfect predictor of your A1C.
While A1C and GMI provide a measure of your glucose management, keep in mind that both are just averages. As the field of diabetes care evolves, many experts are optimistic about the growing use of time in range, which allows for a more reliable picture of daily glucose management and may provide a more meaningful path to better outcomes for people with diabetes.
Learn more about blood sugar metrics here: