The oral glucose-tolerance test (OGTT) is currently the standard method for diagnosis of gestational diabetes (GDM). We conducted a post hoc analysis using the Australian HAPO data to determine seasonal variations in OGTT results and the consequent prevalence of GDM.
Women enrolled in the Australian HAPO study sites (Brisbane and Newcastle) from 2001 to 2006 were included if OGTT results between 24 to 32 weeks gestation were available (n = 2120). Fasting plasma glucose, 1-h plasma glucose, 2-h plasma glucose, HbA1c, and HOMA-IR values were categorized by season and correlated to monthly temperature records from the Australian Bureau of Meteorology for Brisbane and Newcastle. GDM was defined post hoc using the IADPSG/WHO criteria.
Small but significant (p < 0.01 on ANOVA) elevations in fasting glucose (+0.12mM), HbA1c (+0.09%), and HOMA-IR (+0.88 units) were observed during the winter months. Conversely, higher 1-h (+ 0.19mM) and 2-h (+0.33mM) post-load glucose values (both p < 0.01) were observed during the summer months. The correlations between fasting glucose, 1-h glucose, 2-h glucose, and HbA1c with average monthly temperatures confirmed this trend, with positive Pearson’s correlations between 1-h and 2-h glucose with increasing average monthly temperatures, and negative correlations with fasting glucose and HbA1c. Overall prevalence of GDM did not display significant seasonal variations due to the opposing trends seen in the fasting versus 1-h and 2-h post-load values.
A significant winter increase was observed for fasting plasma glucose, HbA1c, and HOMA-IR, which contrasted with changes in1-h and 2-h post-load venous plasma glucose values. While overall prevalence of GDM did not vary significantly by seasons, this study illustrates that seasonality is indeed an additional factor when interpreting OGTT results for the diagnosis of GDM.