The Glucose Management Indicator, or GMI, is a metric providing a clear, objective measure of average glucose control over time. Derived exclusively from data collected by a Continuous Glucose Monitoring (CGM) device, the GMI offers a percentage that reflects the stability and level of blood sugar management. This percentage is designed to give patients and healthcare providers a snapshot of glucose control without needing a blood draw. The GMI serves as a valuable tool for assessing how well a current diabetes management plan is working, offering timely feedback on the effectiveness of diet, exercise, and medication adjustments. It provides a more dynamic and current perspective on glucose trends compared to older, less frequent blood tests.
Defining the Glucose Management Indicator (GMI)
The Glucose Management Indicator is a calculated number representing the mean glucose level observed by a CGM device, mathematically converted into a percentage. This calculation uses an internationally accepted formula derived from studies that compared average glucose readings with traditional blood test results across a large population. The GMI is generated from the average glucose data collected over a period of time, typically requiring at least 10 and preferably 14 or more days of continuous CGM wear.
The percentage is calculated using the mean glucose value from the CGM data, which is then plugged into a specific regression equation. For glucose measured in milligrams per deciliter (mg/dL), the formula is GMI (%) = 3.31 + 0.02392 × [mean glucose in mg/dL]. It is important to note that the GMI is derived only from the interstitial fluid glucose measured by the sensor, not from a laboratory blood test.
This metric was introduced to replace the older term “estimated A1c” (eA1c) to reflect that it is an indicator of glucose management based on CGM data, not a direct prediction of the blood test result. The new name clarifies that the number is an observational metric specific to the CGM data set.
Understanding Target GMI Percentages
The concept of a “normal” GMI percentage is largely based on the established targets for the traditional A1c test, but it is highly individualized by a healthcare provider. For individuals without diabetes, a GMI percentage would typically fall below 5.7%, which correlates with non-diabetic glucose control. A GMI between 5.7% and 6.4% generally suggests prediabetes, indicating average glucose levels are elevated but not yet in the diabetic range.
For most adults living with diabetes, the consensus clinical goal is to maintain a GMI percentage below 7.0%. Achieving this target is associated with a reduced risk of long-term diabetes-related complications. However, this goal is often adjusted based on the patient’s individual health profile and circumstances.
A more stringent target, such as a GMI of less than 6.5%, might be set for patients with a short duration of diabetes or those with a long life expectancy who can achieve this goal without experiencing frequent low blood sugar events. Conversely, a higher target, perhaps up to 7.5% or 8.0%, may be considered appropriate for elderly patients or individuals who have a history of severe hypoglycemia. This personalized approach acknowledges the need to balance the benefits of tight glucose control with the risk of dangerously low blood sugar levels.
GMI vs. HbA1c: Why the Numbers May Not Match
While the GMI and the traditional HbA1c blood test are both reported as a percentage, they are distinct measurements that often produce different numbers for the same person. The HbA1c test measures the percentage of hemoglobin in red blood cells that has glucose attached, or glycated, providing an average glucose level over the red blood cell’s lifespan of approximately three months. The GMI, by contrast, is a calculation based on the average glucose concentration measured in the interstitial fluid beneath the skin by the CGM sensor, typically over a much shorter period.
One reason for a discrepancy is the difference in measurement location; the CGM measures glucose in the tissue fluid, which can lag behind or differ slightly from the glucose concentration in the blood measured by the lab test. More significantly, the traditional A1c test can be affected by factors unrelated to average glucose, such as variations in red blood cell turnover rate. If a person’s red blood cells have a naturally shorter lifespan than the average, their HbA1c may be falsely low compared to their true average glucose, and thus lower than their GMI.
Conditions like anemia, chronic kidney disease (CKD), or certain hemoglobin variants can also interfere with the accuracy of the HbA1c blood test. In such cases, the GMI may be a more reliable indicator of true average glucose exposure over time. Studies have shown that a clinically significant difference of 0.5% or more between GMI and HbA1c occurs frequently, highlighting the importance of using both metrics to get a complete picture of glucose management.
Using GMI for Clinical Decision Making
The GMI serves as a valuable tool in the overall assessment of diabetes management, enabling healthcare teams to make informed decisions about treatment plans. Because GMI is updated frequently with new CGM data, it offers a much faster way to gauge the impact of recent changes to medication, diet, or exercise than waiting three months for a new A1c blood test. If a new therapy is initiated, a change in the GMI within a few weeks provides early feedback on its effectiveness.
The GMI is most effective when used in conjunction with other CGM metrics, particularly Time in Range (TIR). TIR shows the percentage of time a person spends within their target glucose range, typically 70 to 180 mg/dL, which reveals glucose variability that the GMI average alone cannot capture. A GMI that is significantly higher or lower than the patient’s personalized target should prompt a consultation with a healthcare provider to investigate patterns and adjust therapy. Using GMI allows for a more personalized approach to setting and achieving glucose goals.