The glucose management indicator (GMI) is a metric that helps people with diabetes understand the current state of their glucose management.
For many years, doctors relied on A1C blood tests to diagnose diabetes and prediabetes as well as evaluate how well your diabetes treatment is working. While reliable, the A1C test only gives you an understanding of the average blood sugar level over the past two to three months.
GMI evaluates the data over the past 10 to 14 days to estimate the A1C level. Some doctors believe that this metric gives them a better picture of your glucose management efforts.
What is GMI in diabetes management? Let's take a closer look.
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GMI shows what your A1C levels are likely to be based on the average Continuous Glucose Monitoring (CGM) data over the past 10 to 14 days.
If you are wearing a CGM system, it measures glucose in the interstitial (between the cells) fluid every one to five minutes. These numbers can help determine what your A1C level is.
To calculate GMI, you need to take the mean glucose reading over the past 10 to 14 days and convert it from mg/dL to a percentage:
GMI (Percentage) = 3.31 + 0.02392 x (glucose reading in mg/dL)
You need to keep in mind that the accuracy of your GMI depends on the accuracy of your CGM system.
While both GMI and A1C methods measure your blood sugar levels, the figures can differ. A recent study¹ by the University of Washington Diabetes Care Center analyzed 641 people with diabetes and found that the differences between A1C test results and GMI results were mostly minor:
50% of patients had differences of 0.5% or less. For example, if the GMI was 6%, the A1C ranged from 5.5% to 6.5%.
11% of patients had differences of 0.1% or less. For example, if the GMI was 6%, the A1C was between 5.9% and 6.1%.
22% of patients had a difference of 1% or greater. For example, if the GMI was 6%, the A1C was below 5% or greater than 7%.
Since GMI is a fairly new blood sugar level monitoring method, more studies need to be done to prove its accuracy. It's worth noting that one of the reasons GMI was implemented in the first place is problems with A1C accuracy.
The biggest issue with the A1C method is that it measures glucose levels over a long period and then provides an average reading. A good A1C reading doesn't always mean excellent glucose management.
For example, a person could spend one month with low blood sugar levels and one month with high sugar levels. At the end of the two months, the A1C test would show satisfactory average levels. Meanwhile, the person's health could be at serious risk.
Many patients express concern over different numbers in the GMI and A1C reports. It's important to understand that one of the reasons behind the difference is that these methods measure two different things.
People who live with diabetes know that they have to regularly do A1C (also called HbA1C or glycated hemoglobin) tests. Doctors use this common blood test to diagnose pre-diabetes, type 1 diabetes, and type 2 diabetes.
It can also help you understand whether your blood glucose management efforts are working.
The A1C test gives you a figure that reflects average blood sugar levels over the past two to three months. It shows which percentage of hemoglobin proteins (red blood cells that transfer oxygen) in your blood is covered with sugar.
These blood cells last around 120 days, hence the "two to three months" time range.
Several factors can affect the accuracy of your A1C test. They include:
Chronic kidney disease
Certain medicines, including opioids and HIV meds
The higher A1C levels are, the more chances you develop diabetes complications. People with diabetes need to get an A1C test at least two times a year. However, your doctor could recommend doing it more often if you change medication or have trouble keeping glucose levels under control.
GMI takes your CGM values over the past 10 to 14 days, calculates the average figure, and uses it to predict laboratory A1C level. According to recent studies², 10–14 days of CGM data is sufficient to provide a reasonable estimate.
10 days is sufficient for estimating average glucose, time in hyperglycemia (high glucose levels), and time in the target blood glucose range.
14 days or more provides a better estimate for glucose variability and time in hypoglycemia (low glucose levels).
To get GMI readings, you don't need to do a blood test. The data is collected by testing the fluid between your cells.
Since the accuracy of A1C readings depends on many different factors, including the lifespan of your red blood cells, the numbers may not always be accurate. Measuring GMI gives doctors an additional parameter on which to base evaluations and treatments on.
Both A1C and GMI are important metrics. One can't replace another. The difference between these two indicators can provide insights into your condition and glucose management.
For example, you've had satisfactory blood glucose levels over the past three months but suddenly entered a period of acute hyperglycemia due to sickness. If you measure A1C and GMI during that period, the numbers will differ. A1C will be normal, while GMI will be high.
The same issue will occur if you have a period of hypoglycemia. Your A1C will be normal, while your GMI will be low. The difference in these readings can help your doctor understand your condition better and offer timely treatment.
The target for GMI is the same as for A1C. The goal for most adults with diabetes is less than 7%.
Normal – below 5.7%
Prediabetes – between 5.7% and 6.5%
Diabetes – 6.5% and higher
GMI goals can differ depending on many factors, including the patient's life expectancy, chronic diseases, and medication. A doctor can determine what number you should aim for when creating your individual course of treatment.
For example, the ADA advises physicians to consider lowering the A1C target to 6.5% for people with short diabetes duration, long life expectancy, and the absence of serious cardiovascular (heart and blood vessel) diseases.
Some of these people can achieve the goal without adverse treatment effects.
Meanwhile, for people with a history of severe hypoglycemia, short life expectancy, serious macro- or microvascular complications, and extensive comorbidities, the GMI and A1C target can be higher than 8%. This also applies to patients who have a problem achieving A1C goals despite combining the available treatments.
Some doctors suggest that certain patients with type 2 diabetes may need to ignore GMI goals altogether. This includes people with a life expectancy of less than 10 years or those with advanced forms of serious diseases — cancer, emphysema, end-stage kidney failure, etc.
Instead, the treatment should focus on maintaining a satisfactory quality of life.
Another metric that can reflect your glucose level management efforts is Time in Range (TIR). TIR shows the percentage of time you spent in the target glucose range. Similar to GMI and A1C, the metric is presented by a percentage.
Doctors study CGM system readings to understand how much time a person spends in the target glucose range. The parameters you need to know when evaluating your TIR percentage are:
While the target glucose range may differ from one person to another, the typical goal is between 70 and 180 mg/dL.
Most people with type 1 or type 2 diabetes should aim for TIR above 70%. This means they should be in the target glucose range for around 17 hours a day.
A doctor may set lower TIR targets for patients under 25 as well as for older and high-risk patients.
Both TIR and GMI can provide results in real-time, helping doctors and patients understand the full picture. They can be used together with A1C to determine the most effective course of treatment.
While GMI, A1C, and TIR are highly useful indicators, they are still just numbers. It's up to your doctor to evaluate your individual situation and decide on a course of treatment. Attending regular appointments and listening to your doctor is the key to keeping diabetes under control.
Contact your doctor immediately if you start having any symptoms of hyperglycemia or hypoglycemia or your blood sugar levels are high throughout the day.
Glucose management indicator is an important metric that helps people with diabetes and their doctors understand how well the treatment is working. It can also help diagnose prediabetes and diabetes.
When evaluated together with such metrics as A1C and TIR, GMI can be highly useful for glucose level management in people with type 1 and type 2 diabetes.
Using the GMI to estimate your A1C: How accurate is it? | The DiaTribe Foundation
All about your A1C | Centers for Disease Control and Prevention
Executive summary: Standards of medical care in diabetes—2014 | American Diabetes Association
Understanding A1C | American Diabetes Association
Detecting the signs: Hyperglycemia vs. hypoglycemia | The DiaTribe Foundation
Time-in-range and diabetes | Endocrine Society
Glucose management indicator (GMI) | Jaeb Center for Health Research