HbA1c may not correlate with CGM metrics for some adults with type 2 diabetes
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HbA1c and continuous glucose monitoring metrics, such as mean glucose and time in range, may not be correlated for some adults with type 2 diabetes, according to study data. The two measures should be used in a complementary manner, according to researchers.
In findings published in Diabetes Technology & Therapeutics, most adults with type 2 diabetes who achieved a time in range of 70% or higher as measured through CGM did not have an HbA1c of less than 7%, even though mean glucose as measured by CGM was highly correlated with HbA1c.
“HbA1c is the standard measure used to diagnose diabetes and monitor glucose control,” Elizabeth Selvin, PhD, MPH, professor at the Johns Hopkins Bloomberg School of Public Health, told Healio. “CGM is commonly used in type 1 diabetes, and there is growing use in persons with type 2 diabetes; however, most of the evidence for the use of CGM comes from studies of type 1 diabetes. We found that HbA1c was strongly correlated with mean glucose from CGM and with time in range, but there was lots of variation. For some people, mean CGM and HbA1c didn’t line up very well.”
Selvin and colleagues conducted a secondary analysis of data from the Hyperglycemic Profiles in Obstructive Sleep Apnea (HYPNOS) trial, which was conducted to explore how a sleep apnea intervention affected glycemic variability among a cohort of adults with type 2 diabetes. The study included adults aged 21 to 75 years with type 2 diabetes, an HbA1c of 6.5% or higher and untreated obstructive sleep apnea (mean age, 59.7 years; 48.4% women; 55.9% white; 34.4% Black). HbA1c was measured at baseline and 3-month follow-up. Participants wore an Abbott FreeStyle Libre Pro CGM and a Dexcom G4 Platinum CGM simultaneously for up to 14 days. Mean glucose, time in range between 70 mg/dL and 180 mg/dL and time above range with a cutoff of 180 mg/dL and 250 mg/dL were collected.
Mean HbA1c at 3 months was 7.7%. The 4-week mean glucose with both CGM types were similar. Mean glucose was highly correlated with HbA1c for both the Abbott FreeStyle Libre (r = 0.84) and the Dexcom G4 (r = 0.82).
Mean time in range was 70.7% with the Abbott FreeStyle Libre and 72.7% with the Dexcom G4. Time in range was inversely correlated with HbA1c for both the Abbott FreeStyle Libre (r = –0.79) and the Dexcom G4 (r = –0.81). However, among adults who had a time in range of 70% or higher, only 32% to 35% had an HbA1c of less than 7%.
Researchers observed strong correlations between HbA1c and time above range of 180 mg/dL and 250 mg/dL. When the time above range cutoff was 180 mg/dL, 7.1% to 7.7% of participants with time above range of greater than 30% had an HbA1c of less than 7%. No adults had an HbA1c of less than 7% when the time above range cutoff was 250 mg/dL.
“CGM is not a substitute for HbA1c, as some experts have claimed,” Selvin said. “Thus, CGM and HbA1c should be used together in a complementary manner to manage diabetes.”
Selvin noted many studies have explored the strengths and limitations of HbA1c and said similar research must be performed to explore the sources of error within CGM.
For more information:
Elizabeth Selvin, PhD, MPH, can be reached at eselvin@jhu.edu.
Perspective
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Diana Isaacs, PharmD, BCPS, BCACP, BC-ADM, CDCES, FADCES, FCCP
These study results are interesting and put into question the goal of 70% time in range for most people with diabetes as identified by the International Consensus on Time in Range. This study suggests that despite meeting the 70% time in range goal, many people with type 2 diabetes may not actually reach an HbA1c target of less than 7%.
However, an advantage of time in range over HbA1c is that it incorporates glucose variability. In this trial, participants were not treated with insulin and likely experienced less hypoglycemia. Despite achieving the 70% time in range, the lower rates of hypoglycemia could have contributed to a higher average glucose and higher HbA1c. One could also argue that if a person can achieve 70% time in range safely without any hypoglycemia, then why not aim for a higher time in range, such as 80% or 90%? Or even narrowing the target from a range of 70 mg/dL to 180 mg/dL to a target range of 70 mg/dL to 140 mg/dL, especially for people not treated with insulin.
Ultimately, we need more analyses of what the ideal time in range targets should be for people with type 2 diabetes and how the targets impact microvascular and macrovascular outcomes.
In the interim, I agree with the author’s conclusion to use the HbA1c in conjunction with the CGM key metrics. Because average CGM glucose had a stronger correlation with HbA1c, it would be reasonable to use average CGM glucose and glucose management indicator as other important metrics to follow.
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