Translating HbA(1c) measurements into estimated average glucose values in pregnant women with diabetes

AIMS/HYPOTHESIS: This study aimed to examine the relationship between average glucose levels, assessed by continuous glucose monitoring (CGM), and HbA(1c) levels in pregnant women with diabetes to determine whether calculations of standard estimated average glucose (eAG) levels from HbA(1c) measurements are applicable to pregnant women with diabetes. METHODS: CGM data from 117 pregnant women (89 women with type 1 diabetes; 28 women with type 2 diabetes) were analysed. Average glucose levels were calculated from 5–7 day CGM profiles (mean 1275 glucose values per profile) and paired with a corresponding (±1 week) HbA(1c) measure. In total, 688 average glucose–HbA(1c) pairs were obtained across pregnancy (mean six pairs per participant). Average glucose level was used as the dependent variable in a regression model. Covariates were gestational week, study centre and HbA(1c). RESULTS: There was a strong association between HbA(1c) and average glucose values in pregnancy (coefficient 0.67 [95% CI 0.57, 0.78]), i.e. a 1% (11 mmol/mol) difference in HbA(1c) corresponded to a 0.67 mmol/l difference in average glucose. The random effects model that included gestational week as a curvilinear (quadratic) covariate fitted best, allowing calculation of a pregnancy-specific eAG (PeAG). This showed that an HbA(1c) of 8.0% (64 mmol/mol) gave a PeAG of 7.4–7.7 mmol/l (depending on gestational week), compared with a standard eAG of 10.2 mmol/l. The PeAG associated with maintaining an HbA(1c) level of 6.0% (42 mmol/mol) during pregnancy was between 6.4 and 6.7 mmol/l, depending on gestational week. CONCLUSIONS/INTERPRETATION: The HbA(1c)–average glucose relationship is altered by pregnancy. Routinely generated standard eAG values do not account for this difference between pregnant and non-pregnant individuals and, thus, should not be used during pregnancy. Instead, the PeAG values deduced in the current study are recommended for antenatal clinical care. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00125-017-4205-7) contains peer-reviewed but unedited supplementary material, which is available to authorised users.