The discordance between HbA1c and glucose tolerance testing for the postpartum exclusion of diabetes following gestational diabetes

Aims: To assess the concordance between the HbA1c and the oral glucose tolerance test (OGTT) for the diagnosis of diabetes and prediabetes following gestational diabetes (GDM) in an ethnically diverse population.

Methods: Women with GDM underwent a concurrent OGTT and HbA1c test 6-12 weeks postpartum.

Results: There were 114 women with GDM who had a 75 g 2-h OGTT and HbA1c at 9.0 ± 3.2 weeks postpartum. Five subjects had diabetes using OGTT criteria, and 4 by HbA1c criteria. No subjects had diabetes on both criteria. The overall concordance between the OGTT and HbA1c for the diagnosis of diabetes, prediabetes, or normal glucose tolerance was only 54% (κ coefficient 0.058, p=0.41). Gravidity, the 2-h glucose level on the OGTT during pregnancy, and the 3rd trimester HbA1c predicted discordance between the postpartum OGTT and HbA1c.

Conclusions: There is poor concordance between the OGTT and HbA1c for the diagnosis of diabetes and prediabetes following GDM. This reflects that the two tests measure different aspects of dysglycemia. In the post-GDM population, the HbA1c misses cases of diabetes as identified by the OGTT. We recommend that the OGTT be retained for postpartum diabetes testing following GDM.