CGM measures have variable correlation with HbA1c
medwireNews: Glycated hemoglobin (HbA1c) levels correlate strongly with continuous glucose monitoring (CGM)-measured average blood glucose and hyperglycemia, but poorly with hypoglycemia, say researchers.
“The present analysis adds detail to the close relationship that has been observed between several CGM-derived glycaemic variables and HbA1c, with an especially strong correlation between HbA1c and mean glucose,” John Welsh (Dexcom Inc, San Diego, California, USA) and colleagues write in Diabetic Medicine.
By contrast, although the percentage of measurements below 3.9 mmol/mol (70 mg/dL) fell from 5.6% when average HbA1c values were below 48 mmol/mol (6.5%) to 1.1% when they were at least 69 mmol/mol (8.5%), the overall association between HbA1c and hypoglycemia was weak.
Indeed, among people with these higher HbA1c values, the time in hypoglycemia ranged from approximately 0% to 6%, and the team highlights that glucose values below 3.0 mmol/L (54 mg/dL) were also noted in this subgroup, “showing that relaxing HbA1c goals is an ineffective strategy for hypoglycaemia prevention.”
“These results confirm prior studies showing that people with similar HbA1c levels may have widely disparate exposure to hypoglycaemia and hyperglycaemia, and emphasize the value of CGM studies when evaluating people with diabetes,” says the team.
The findings are based on data from 530 people with type 1 or insulin-dependent type 2 diabetes who wore a Dexcom G4 or G5 CGM device in one of four randomized controlled trials. The available data amounted to approximately 30 days of CGM per participant.
Correlations between HbA1c and other CGM measures were much stronger. Every 10% change in the time that people spent within the blood glucose range of 3.9–10.0 mmol/L (70–180 mg/dL) was associated with a 7 mmol/mol (0.7%) change in HbA1c, and average blood glucose and the percentage of glucose measurements greater than 13.9 mmol/L (250 mg/dL) were also significantly associated with HbA1c.
But despite these close associations, there was relatively wide variability in the average HbA1c value seen for a given CGM value, and vice versa. These findings “confirm and extend earlier observations of individual variation in glycation ratios and justify incorporating CGM-derived metrics into routine care discussions,” conclude the researchers.
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