Most patients with type 1 diabetes will experience clinically meaningful differences in glucose level results when comparing laboratory-measured hemoglobin A1c (HbA1c) levels with HbA1c levels derived from a glucose management indicator (GMI), an updated approach to using data from continuous glucose monitoring. These differences should be considered when determining appropriate glucose management goals, according to an e-letter published in Diabetes Care.

GMI (%) is calculated using the formula GMI = 3.31 + 0.02392 × mean glucose in mg/dL, which is derived by regressing contemporaneously measured HbA1c values against mean sensor glucose levels. The study investigators sought to compare laboratory HbA1c results estimated from continuous glucose monitoring data with HbA1c calculated using GMI. GMI was assessed using data from 3 12-week randomized controlled trials of the glucose sensors Navigator 2 (n = 114) and Guardian 3 (n = 85), as well as data from Dexcom sensors. Trial participants were children and adults with type 1 diabetes and HbA1c levels between 7.5% and 10%.

The percentage of participants with similar results for both GMI and laboratory HbA1c was comparable for all sensors, including Dexcom sensors (19% to 20%). The percentage of participants with >0.5% deviation between GMI and laboratory HbA1c was 36% for Navigator 2, 32% for Guardian 3, and 28% for Dexcom, with substantially overlapping confidence intervals. Across all 3 sensor types, there was a substantial number of participants with clinically meaningful differences between laboratory HbA1c and glucose sensor-derived HbA1c using GMI. As such, the data validate the use of the GMI formula with Navigator 2 and Guardian 3, wrote the researchers.

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The investigators included that the differences found through alternate types of HbA1c measurement should be carefully considered by clinicians and patients when personalizing glucose management approaches.

“[I]f a person has a GMI always considerably lower than expected from measured HbA1c, one has to be careful not to set the therapeutic goal based on the laboratory HbA1c target too low and to ensure that time spent in hypoglycemia is not excessive,” concluded the researchers. “Further studies with larger databases using these and other sensors will be helpful to inform clinicians how meaningful the difference in laboratory HbA1c versus GMI is when aiming to personalize HbA1c for diabetes management.”

Some investigators reported financial connections, such as research support and consulting fees, to the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.

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Leelarathna L, Beck RW, Bergenstal RM, Thabit H, Hovorka R. Glucose management indicator (GMI): insights and validation using Guardian 3 and Navigator 2 sensor data [published online February 6, 2019]. Diabetes Care. doi:10.2337/dc18-2479