The GMI Spells A New Way to Evaluate Your Glucose Numbers
By Dr. Laurel Messer
Though A1C has long been used as the primary metric for evaluating someone’s estimated average blood sugar, the glucose management indicator (GMI) is an alternative that can provide the same insights without the need for a blood draw. It can also provide this information for much shorter time periods to evaluate lifestyle changes, illnesses, and new medications
For the past four decades, A1C and fingerstick blood sugars have been used to assess diabetes management. A1C (or hemoglobin A1C) measures how much sugar is bound to proteins in red blood cells (called glycated hemoglobin) and provides an estimated measure of glucose levels over a long period of time – the two to three months that red blood cells survive in our bodies.
The A1C does have limitations but is a number that people with diabetes and healthcare professionals are comfortable with and understand how to interpret. The purpose of this article is to show that the Glucose Management Indicator (GMI) is another measure for glycemic management and can be interpreted in a similar way to A1C. GMI is one more tool for the diabetes toolbox – click to learn about another tool, Time in Range.
GMI used to be called the “estimated A1C” or the “eA1C,” because it is meant to approximate an A1C lab value. Instead of being determined by how much sugar is attached to hemoglobin molecules on red blood cells, GMI is calculated directly from the mean (or average) glucose value from a continuous glucose monitoring device (CGM). This means that GMI is a more direct and more accurate measure of glucose levels – it can be calculated for a shorter period of time (the ideal minimum is two weeks), without the need to wait two to three months and to have a laboratory blood draw. Any person wearing a CGM can calculate their GMI in a matter of seconds if they have enough glucose values, and many CGM devices automatically do the calculation using your mean glucose; the calculator can be found here.
To be clear: A GMI can only be calculated if you have a CGM, but if you do have that device, here are the advantages of using GMI.
In addition to providing an additional assessment of glucose values, GMI eliminates misinterpretation of the variance in A1C caused by a number of factors including kidney disease, iron deficiency, sickle cell disease, the use of certain medications, and differences between racial and ethnic groups. Two people could have identical glucose data but different A1C levels depending on their rates of red blood cell survival and glucose attachment to proteins (referred to as glycation), with much more variation within races than between races. However, these same two people would have the same calculated GMI, giving them a more accurate and consistent understanding of their diabetes management and their risk of health complications.
The best feature of GMI (compared to A1C) is that you can (and should!) choose different periods of time to calculate. For example, you can calculate a GMI for the past two weeks, which you cannot do with an A1C, since A1C correlates to glucose levels over the preceding two to three months. Have you ever had an A1C level affected by one week of illness with very high glucose levels? For many people who receive an A1C level at clinical visits two to four times a year, this can be frustrating, as it highlights a time of high glucose values rather than the overall glucose management for that period. Calculating a GMI can be helpful with illness: a clinician or person with diabetes can select a period of time (for example, two weeks) before or after the illness to get a better sense of overall glycemic management that includes or eliminates the period of illness. Time in Range is another helpful metric for understanding glucose levels during a distinct (and even shorter!) time period.
GMI can also be useful to assess the impact of a lifestyle change, like a new diet or exercise regimen, or a medication adjustment. For example, a person could calculate GMI for a two-week period, change diet to lower carbohydrates, and calculate another GMI for the two weeks after the change. This would give a more accurate and immediate indicator of the effect of the change, compared to an A1C measurement drawn at a clinic that cannot focus on the time in question. Another example would be starting a new medication that affects glucose levels. In addition to assessing daily trends in glucose levels, a GMI after one to two weeks on a new medication can provide valuable information on whether medication doses need to be changed.
Finally, in the time of COVID-19 and increased telehealth, GMI is the perfect substitute for a lab-drawn A1C. No blood draw and no in-person clinic visit is required. That said, remember that your GMI may run higher or lower than your lab-drawn A1C for many reasons discussed above – like red blood cell turnover, health conditions, medications, and more.
We encourage you to discuss the use of GMI and CGM with your diabetes healthcare team. GMI can be interpreted the same way as an A1C, but for the reasons stated above, it can also be more accurate and can be used as a tool to determine the effect of illness, lifestyle changes, and medication adjustments. We also hope to help diabetes care professionals become more comfortable with GMI and other CGM-derived glucose metrics, which take in the whole picture of daily glucose control. Next step, Time in Range! (GMI can bridge A1C and Time in Range – but that is another story.)
This article is part of a series on Time in Range.
The diaTribe Foundation, in concert with the Time in Range Coalition, is committed to helping people with diabetes and their caregivers understand Time in Range to maximize patients' health. Learn more about the Time in Range Coalition here.