Healthcare providers must evaluate both HbA1c and continuous glucose monitoring (CGM) metrics when managing diabetes, as neither metric alone can fully capture a patient’s glycemic control. This was the key message shared by experts during a presentation at the Association of Diabetes Care and Education Specialists (ADCES) annual meeting.
Dr. Viral N. Shah, a professor of medicine at Indiana University, and Dr. J. Daniel Ruck, a doctor of nursing practice at Joslin Diabetes Center, emphasized that relying on a single metric can be misleading. Instead, they advocated for a comprehensive assessment of all available data, tailored to the individual needs of each patient.
“We are treating an individual with diabetes, not just numbers,” Dr. Shah explained. “Therefore, it’s essential to consider all different numbers, such as HbA1c and CGM metrics, in light of person-centric goals.”
The Evolu tion of HbA1c
Since the Diabetes Control and Complications Trial (DCCT) in 1993, HbA1c has been regarded as the gold standard for assessing the risk of diabetes complications. However, while HbA1c remains valuable in research, its utility in everyday diabetes management is limited.
“On the ground, HbA1c isn’t as powerful for immediate diabetes management,” Dr. Ruck noted. “Various factors can influence HbA1c measurements, including certain rare blood disorders.”
Furthermore, HbA1c does not provide detailed insights into a patient’s hypoglycemic or hyperglycemic episodes, nor does it reveal the overall glucose profile, making it less effective for fine-tuning daily management.
Time in Range: An Evolving Metric
CGM offers a more detailed view of glucose fluctuations, but it too has its limitations. Time in range, initially defined as glucose levels between 70 mg/dL and 180 mg/dL, has been a central metric for assessing glycemic control. However, Dr. Shah pointed out that the upper limit of 180 mg/dL was based on the postprandial glucose target for most people with diabetes at a time when fewer treatment options were available.
“Given that we define prediabetes by a glucose level of more than 140 mg/dL, it makes more sense to set the upper limit for time in range at 140 mg/dL,” Dr. Shah suggested. This lower range may be more appropriate for individuals with an HbA1c below 6.5%, those with prediabetes, and people with type 2 diabetes using noninsulin therapies.
Dr. Ruck added that time in range goals should be personalized, considering the individual’s safety and risk of hypoglycemia. “The challenge has always been balancing tight glucose control with the risk of low blood sugar,” he said. “If we can achieve tighter control without increasing that risk, it’s a worthy goal.”
Understanding Hypoglycemia and GMI
Time below range, or hypoglycemia, is another critical CGM metric, with its own set of challenges. Initially defined in 2005 as a glucose level below 70 mg/dL, this threshold has since been refined. The American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) now categorize hypoglycemia into three levels: level 1 (70 mg/dL or less), level 2 (54 mg/dL or less), and level 3 (severe hypoglycemia requiring external assistance).
Dr. Shah highlighted that expressing time below range as a percentage rather than the number of events can be less meaningful for both patients and healthcare providers. Moreover, the relevance of level 1 hypoglycemia is limited for people with prediabetes and type 2 diabetes who are not on insulin or sulfonylurea therapies.
Glucose management indicator (GMI), an estimated HbA1c derived from CGM data, is another metric with limitations. GMI is calculated using data from clinical trials, but it may overestimate HbA1c in people with lower HbA1c levels and underestimate it in those with higher levels.
“GMI is a mathematical estimation, while HbA1c reflects physiological glycation,” Dr. Shah explained, “which means GMI may not correspond precisely to HbA1c for everyone.”
Given the limitations of both HbA1c and CGM metrics, the speakers urged healthcare providers to adopt a holistic approach to diabetes management, considering all available data and tailoring treatment plans to the unique needs of each patient.
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