The Relationships Between Time in Range, Hyperglycemia Metrics, and HbA1c

• Published in: Journal of diabetes science and technology Vol. 13; no. 4; pp. 614 - 626
• Main Authors: Beck, Roy W., Bergenstal, Richard M., Cheng, Peiyao, Kollman, Craig, Carlson, Anders L., Johnson, Mary L., Rodbard, David
• Format: Journal Article
• Language: English
• Published: Los Angeles, CA SAGE Publications 01-07-2019
• Subjects: Special Section: Interpreting CGM Data; continuous glucose monitoring; type 1 diabetes; glucose time in range
• ISSN: 1932-2968; 1932-2968; 1932-3107
• DOI: 10.1177/1932296818822496

Background:
As the use of continuous glucose monitoring (CGM) increases, there is a need to better understand key metrics of time in range 70-180 mg/dL (TIR70-180) and hyperglycemia and how they relate to hemoglobin A1c (A1C).
Methods:
Analyses were conducted utilizing datasets from four randomized trials encompassing 545 adults with type 1 diabetes (T1D) who had central-laboratory measurements of A1C. CGM metrics were calculated and compared with each other and A1C cross-sectionally and longitudinally.
Results:
Correlations among CGM metrics (TIR70-180, time >180 mg/dL, time >250 mg/dL, mean glucose, area under the curve above 180 mg/dL, high blood glucose index, and time in range 70-140 mg/dL) were typically 0.90 or greater. Correlations of each metric with A1C were lower (absolute values 0.66-0.71 at baseline and 0.73-0.78 at month 6). For a given TIR70-180 percentage, there was a wide range of possible A1C levels that could be associated with that TIR70-180 level. On average, a TIR70-180 of 70% and 50% corresponded with an A1C of approximately 7% and 8%, respectively. There also was considerable spread of change in A1C for a given change in TIR70-180, and vice versa. An increase in TIR70-180 of 10% (2.4 hours per day) corresponded to a decrease in A1C of 0.6%, on average.
Conclusions:
In T1D, CGM measures reflecting hyperglycemia (including TIR and mean glucose) are highly correlated with each other but only moderately correlated with A1C. For a given TIR or change in TIR there is a wide range of possible corresponding A1C values.