Time-in-range (TIR) readings from a continuous glucose monitor (CGM) complemented A1c readings and provided a more complete picture of glucose control in patients with type 2 diabetes taking basal insulin, in a post-hoc analysis of the SWITCH PRO clinical trial.
TIR was inversely related to A1c, with the strongest correlation following treatment intensification.
However, "there was a wide scatter of data, indicating that TIR (and other metrics) provides information about glycemic control that cannot be discerned from A1c alone, and which at least complements it," Ronald M. Goldenberg, MD, from LMC Diabetes & Endocrinology in Thornhill, Ontario, Canada, and colleagues write in their article published in the May issue of Diabetes Therapy.
Other work has shown that more than a third of people with type 2 diabetes are not achieving the internationally recommended A1c target of < 7% to 8.5%, they note.
When used with A1c, CGM data — such as TIR, time below range (TBR), and time above range (TAR) — "provide a more complete picture of glucose levels throughout the day and night," they write.
"This may help empower people with diabetes to better manage their condition, giving them practical insights into the factors driving daily fluctuations in glucose levels, such as diet, exercise, insulin dosage, and insulin timing," they add. "These metrics may also be used to inform treatment decisions by healthcare professionals."
"Ultimately," the researchers conclude, "it is hoped that the use of these new metrics should lead to an improved quality of glycemic control and, in turn, to a reduction in the number of diabetes-related complications."
'Important Study'
Invited to comment, Celest C. Thomas, MD, who was not involved with the research, said: "This study is important because it is consistent with previous analyses that found a correlation between TIR and A1c."
But "I was surprised by the scatter plots which identified participants with TIR of 70% that also had A1c > 9%," she added, in an email to Medscape Medical News. "This highlights the importance of using multiple glycemic metrics to understand an individual's risk for diabetes complications and to be aware of the limitations of the metrics."
Thomas, from the University of Chicago, Illinois, also noted that CGM is used in endocrinology clinics and increasingly in primary care clinics, "often to determine glycemic patterns to optimize therapeutic management but also to review TIR and, importantly, time below range to reduce the incidence of hypoglycemia."
And people with type 2 diabetes are using CGM, Thomas noted, to understand their individual responses to medications, food choices, sleep quality and duration, exercise, and other day-to-day variables that impact glucose levels. "In my clinical practice, the information provided by personal CGM is empowering," she said.
Effective April 4, 2023, Medicare "allows for the coverage of CGM in patients [with type 2 diabetes] treated with one injection of insulin daily and those not taking insulin but with a history of hypoglycemia," Thomas noted, whereas "previously, patients needed to be prescribed at least three injections of insulin daily. Other insurers will hopefully soon follow."
"I foresee CGM and TIR being widely used in clinical practice for people living with type 2 diabetes," she said, "especially those who have ever had an A1c over 8%, those with a history of hypoglycemia, and those treated with medications that are known to cause hypoglycemia."
How Does TIR Compare With A1c?
Goldenberg and colleagues set out to better understand how the emerging TIR metric compares with the traditional A1c value.
They performed a post-hoc analysis of data from the phase 4 SWITCH PRO study of basal insulin-treated patients with type 2 diabetes with at least one risk factor for hypoglycemia.
The patients were treated with insulin degludec or glargine 100 during a 16-week titration and 2-week maintenance phase, and then crossed over to the other treatment for the same time periods.
Glycemic control was evaluated using a blinded professional CGM (Abbott Freestyle Libro Pro). The primary outcome was TIR, which was defined as the percentage of time spent in the blood glucose range of 70 to 180 mg/dL.
There were 419 participants in the full analysis. Patients were a mean age of 63 and 48% were men. They had a mean body mass index of 32 kg/m2 and had diabetes for a mean of 15 years.
There was a moderate inverse linear correlation between TIR and A1c at baseline, which became stronger following treatment intensification during the maintenance periods in the full cohort, and in a subgroup of patients with median A1c ≥ 7.5% (212 patients).
This correlation between TIR and A1c was poorer in the subgroup of patients with baseline median A1c < 7.5% (307 patients).
The data were widely scattered, "supporting the premise that A1c and TIR can be relatively crude surrogates of each other when it comes to individual patients," Goldenberg and colleagues note.
Where individual patients have both low A1c and low TIR values, this might indicate frequent episodes of hypoglycemia.
A few individual patients had TIR > 70% but A1c approaching 9%. These patients may have different red blood cell physiology whereby A1c does not reflect average glycemic values, the researchers suggest.
The study was sponsored by Novo Nordisk and several authors are Novo Nordisk employees. The complete author disclosures are listed with the article. Thomas has reported no relevant financial relationships.
Diabetes Ther. 2023;14:915-924. Full text
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Cite this: CGM Completes Picture of A1c in Type 2 Diabetes - Medscape - May 12, 2023.
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