The glucose challenge test in pregnancy identifies future risk of diabetes

Context — Women with gestational diabetes (GDM) have an increased future risk of type 2 diabetes but, in practice, their recommended postpartum glucose tolerance testing is often missed or substituted with measurement of A1c instead.

Objective — We hypothesized that the antenatal screening glucose challenge test (GCT) should predict future diabetes risk and, if so, would have thresholds that identify the same degree of risk as the diagnosis of prediabetes on postpartum measurement of A1c.

Methods — With population-based administrative databases, we identified all women in Ontario, Canada, who had a GCT in pregnancy with delivery between January 2007 and December 2017, followed by measurement of A1c and fasting glucose within 2 years postpartum (n = 141 858, including 19 034 with GDM). Women were followed over a median of 3.5 years for the development of diabetes.

Results — Under the assumption of a linear exposure effect, the 1-hour post-challenge glucose concentration on the GCT was associated with an increased likelihood of developing diabetes (hazard ratio 1.39; 95% CI, 1.38-1.40). A GCT threshold of 8.0 mmol/L predicted the same 5-year risk of diabetes (6.0%; 95% CI, 5.8-6.2) as postpartum A1c 5.7% (identifying prediabetes). Moreover, in women with GDM, a GCT threshold of 9.8 mmol/L equaled prediabetes on postpartum A1c in predicting a 5-year risk of diabetes of 16.5% (14.8-18.2).

Conclusion — The GCT offers predictive capacity for future diabetes in pregnant women. In women with GDM, this insight could identify those at highest risk of diabetes, toward whom postpartum screening efforts should be most strongly directed.