Circadian Variation in the Response to the Glucose Challenge Test in Pregnancy: Implications for screening for gestational diabetes mellitus

OBJECTIVE: A common approach to screening for gestational diabetes mellitus (GDM) is the universal testing of all pregnant women with a 1-h, 50-g glucose challenge test (GCT), followed by a diagnostic oral glucose tolerance test (OGTT) in those in whom the GCT is positive (≥7.8 mmol/L). More important, the GCT is performed at any time of day, but there has been limited study of the effect of time of day on test performance. Thus, using their subsequent OGTT (performed in the morning), we sought to characterize the metabolic function of women with positive GCTs in relation to the timing of their test. RESEARCH DESIGN AND METHODS: A total of 927 women with positive GCTs underwent a 3-h 100-g OGTT. They were stratified into four groups by time of day (hours) of their GCT: <0900 (n = 171), 0900–1059 (n = 288), 1100–1259 (n = 189), and ≥1300 (n = 279). RESULTS: On the OGTT, the prevalence of GDM progressively decreased across the GCT groups from <0900 h (26.9%) to 0900–1059 h (25.0%) to 1100–1259 h (21.7%) to ≥1300 h (21.5%; P = 0.0022). After adjustment for GDM risk factors, mean adjusted glucose area under the curve (AUC(gluc)) similarly decreased across the groups, while insulin sensitivity (Matsuda index) and β-cell function (Insulin Secretion-Sensitivity Index-2) progressively increased (all P < 0.0001). In particular, compared with the <0900- and 0900–1059-h groups, women whose positive GCT occurred after 1300 h had superior metabolic function, as evidenced by lower AUC(gluc), higher insulin sensitivity, and better β-cell function (all P ≤ 0.0097). CONCLUSIONS: Among women with a positive GCT, those tested in the afternoon have better metabolic function and a lower risk of GDM on subsequent OGTT.