Small variations in fasting blood glucose have significant effects in diagnosis of gestational diabetes mellitus

Background: Gestational diabetes mellitus (GDM) is defined as any degree of glucose intolerance with onset during pregnancy. The correspondence between GDM cases ascertained in the first trimester (<12 weeks) using fasting blood glucose (FG) levels (FG >5.1 mmol/L) and third trimester (24-28 weeks) using Hyperglycemia Adverse Pregnancy Outcome (HAPO) study guidelines is not perfect and vary according to the cutoffs used. This retrospective observational study analyzed the variation in the prevalence of GDM for different cutoffs of FG in the first and third trimester. Methods: We analyzed registers of FG during early (<12 weeks; n=38,489) and late (24-28 weeks; n=60,432) stages of gestation. The estimated prevalence of GDM was calculated for both periods by selecting women with FG values between 5.0 to 5.3 mmol/L (90 to 95 mg/dL, HAPO study, odds ratio 1.5 to 2.0). The dispersion of the recommended cut-off by HAPO (OR 1.75) to GDM diagnosis, 5.1 mmol/L (92 mg/dL) was calculated based on the analytical (CVa 2%), biological (CVi 5.0%), and reference change value (RCV). Results: For early and late stages of pregnancies, the estimated GDM frequencies were 13.4% to 5.6% and 14.7% to 10.6%, respectively, with the FG cut-off varying from 5.0 to 5.3 mmol/L. The estimated variations of 5.1 mmol/L with (CVa 2% + CVi 5%), were 4.8 to 5.4 mmol/L. RCV, applied to a 5.1 mmol/L decision point showed a variation of 4.3 to 5.9 mmol/L. The 0.055 mmol/L (1 mg/dL) variation in fasting glycemia for the cut-off criterion of 5.1 mmol/L resulted in variations for estimated GDM of about 7% (+7% to -6%) of cases in late stages and about 15% (+19% to -15%) in the initial stages. Conclusions: Fasting glycemia is linked to the diagnosis of GDM and presents a wide variation, mostly derived from intra-individual biological variability (CVi). Small variations of FG cut-off from an initial value (5.1 mmol/L) change the prevalence of GDM more expressively in early stages of pregnancy.