Pregnancy-specific glucose to HbA1c conversion is essential
medwireNews: The relationship between average blood glucose levels and glycated hemoglobin (HbA1c) is altered during pregnancy, show findings based on continuous glucose monitoring (CGM).
Using data from 688 women with type 1 or type 2 diabetes who participated in two studies of CGM in pregnancy, the researchers found that, as in nonpregnant diabetes patients, there was a strong positive association between blood glucose and HbA1c levels.
However, the average blood glucose change associated with a 1% change in HbA1c was 0.67 mmol/L, compared with a reported 1.0–2.0 mmol/L change in nonpregnant diabetes patients.
“This suggests that a change in HbA1c during pregnancy reflects a smaller change in average glucose compared with that seen outside of pregnancy,” write Eleanor Scott (University of Leeds, UK) and study co-authors in Diabetologia.
They note that an altered glucose–HbA1c relationship has long been suspected, leading “key bodies” to advise against using HbA1c as a marker of average blood glucose during pregnancy. Yet it is reportedly widely used, says the team, with anecdotal evidence also suggesting that clinicians are using the standard model for estimating average glucose from HbA1c, “despite it being derived from data from non-pregnant adults.”
Furthermore, the relationship between blood glucose and HbA1c changed according to the week of pregnancy. For example, using the formula Scott and team developed based on their best-fitting model, an HbA1c of 6.0% equated to an average blood glucose level of 6.7 mmol/L at week 12 but of 6.4 mmol/L at week 36.
Overall, the researchers’ findings suggest that maintaining blood glucose levels of 6.4–6.7 mmol/L, estimated according to their formula, throughout pregnancy would achieve an HbA1c of 6.0%, which they note “is necessary for reducing the risk of adverse pregnancy outcomes.”
The standard conversion formula, from the A1C-Derived Average Glucose study, would produce a blood glucose value of 7 mmol/L for an HbA1c of 6.0%, regardless of gestational week, the team notes, with the difference becoming more pronounced as HbA1c increased.
“This means that pregnant women and their clinicians could be misled by the standard [estimated average glucose] readings currently generated for laboratory reports and by automated online calculators,” Scott et al conclude.
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