Providers should assess HbA1c and CGM metrics in diabetes management
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Key takeaways:
- Time in range, time below range and glucose management indicator are three CGM metrics with limitations.
- Providers should consider HbA1c and CGM metrics while caring for a person with diabetes.
Both HbA1c and continuous glucose monitoring metrics have limitations, and providers should not rely on a single metric for managing a patient’s diabetes, according to two speakers.
During a presentation at the Association of Diabetes Care and Education Specialists annual meeting, Viral N. Shah, MD, professor of medicine in the division of endocrinology and metabolism at Indiana University, and J. Daniel Ruck, DNP, BC-ADM, CDCES, doctor of nursing practice at Harvard Medical Faculty Physicians at Joslin Diabetes Center in Boston, discussed how no single metric to assess glycemic control in diabetes is perfect. They recommended providers examine all data points they have for a patient and to consider each individual’s situation when managing diabetes.
“We are treating an individual with diabetes and not the numbers,” Shah told Healio. “Hence, all different numbers, HbA1c and CGM metrics, should be taken together in account considering person-centric views and goals.”
Publication of the Diabetes Control and Complications Trial (DCCT) in 1993 established HbA1c as the gold standard for assessing risk for diabetes complications, according to Ruck. HbA1c has value in research, but Ruck said it is less useful for day-to-day diabetes management.
“When we talk about on the ground what we can do tomorrow [for diabetes management], HbA1c is not as powerful,” Ruck said during the presentation. “Many factors can influence HbA1c measurements. If you have got a rare blood disease, that can affect it.”
Additionally, HbA1c does not provide information on hypoglycemia, hyperglycemia or the overall glucose profile of a person with diabetes, according to Ruck.
Time in range limitations
CGM can provide more data an HbA1c measurement, but Shah noted that CGM data also have limitations. In a paper published in Journal of Diabetes Science and Technology in 2013, time in range was established as a glucose between 70 mg/dL and 180 mg/dL. Shah said 180 mg/dL was defined as the upper limit of time in range at the time because it was the postprandial glucose goal for most people with diabetes and there were no effective therapies to keep glucose within a target range of 70 mg/dL to 140 mg/dL.
“We define prediabetes by glucose on an oral glucose tolerance test of more than 140 mg/dL,” Shah said during the presentation. “So 140 mg/dL has been accepted because it does increase the risk for cardiovascular disease independent of progression to diabetes. That limit of 140 mg/dL makes more sense to define the time in range, which now we are going to call time in right range.”
Shah said a time in range of 70 mg/dL to 140 mg/dL is a more physiological target for people with an HbA1c of less than 6.5%, those with prediabetes and people with type 2 diabetes using a GLP-1 receptor agonist or other noninsulin therapy. Ruck said providers should set time in range goals based on each individual’s situation.
“How safely we can target specific goals should be considered,” Ruck told Healio. “The enemy of tight blood sugar control has always been low blood sugar, and if we can get people into tighter range without increasing their low blood sugar risk, then I see this as a good goal to pursue.”
Hypoglycemia, GMI limitations
Limitations also exist with time in hypoglycemia, or time below range. Time below range was defined as 70 mg/dL based on a 2005 report from an American Diabetes Association working group and was later accepted as the hypoglycemia cutoff by the FDA for artificial pancreas studies. Shah said the ADA working group decision was based off of hypoglycemic clamp studies done among people without diabetes and those with type 1 diabetes and generally did not include those with higher HbA1c measurements.
In 2016, a joint position statement from the ADA and European Association for the Study of Diabetes revised the hypoglycemia cutoffs. The paper defined level 1 hypoglycemia as a “glucose alert value” of 70 mg/dL or less. Level 2 hypoglycemia was considered glucose of 54 mg/dL or less as detected by self-monitoring of plasma glucose, laboratory measurement of plasma glucose or CGM. Level 3 hypoglycemia was defined as severe hypoglycemia with severe cognitive impairment and requiring external assistance for recovery.
“The current limitation of time below range is that it is expressed as a percentage instead of events, and events are more meaningful to patients as well as payers,” Shah told Healio. “Also, level 1 time below range may not be very relevant for people with prediabetes and type 2 diabetes who are not on insulin or sulfonylurea.”
Shah also discussed limitations with glucose management indicator (GMI), which is an estimated HbA1c based on CGM data. It is calculated using a formula that was developed from data in the REPLACE-BG, DIAMOND-T1D and DIAMOND-T2D clinical trials. Shah said the mean HbA1c of the population was 7.3%, meaning GMI may overestimate HbA1c in people with an HbA1c of less than 6.5%, and underestimate the figure in people with an HbA1c more than 10%.
“GMI is a mathematic estimation of HbA1c, while HbA1c is physiological glycation and hence GMI is not necessarily equal to HbA1c for all people with diabetes,” Shah told Healio.
Shah said the limitations of HbA1c and CGM metrics mean providers need to individualize care and take all data into account when managing a person’s diabetes.
References:
- Bergenstal RM, et al. J Diabetes Sci Technol. 2013;doi:10.1177/193229681300700234.
- International Hypoglycaemia Study Group. Diabetes Care. 2017;doi:10.2337/dc16-2215.
- Workgroup on Hypoglycemia, American Diabetes Association. Diabetes Care. 2005;doi:10.2337/diacare.28.5.1245.
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Shah V, et al. Abstract F09A. Presented at: ADCES24; Aug, 9-12, 2024; New Orleans.
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