Objectives: To determine which of the American Diabetes Association (ADA) or World Health Organization (WHO) plasma glucose criteria for gestational diabetes mellitus (GDM) best predicts poor fetal outcome. To determine whether an alternative cut-off point would result in increased predictive value and greater diagnostic effectiveness in pregnancies at high risk for GDM.
Research design and methods: A sample of 473 successive apparently normal pregnant women attending the Obstetric Department were screened for GDM using both the ADA and the WHO criteria. Between 26 and 30 weeks of gestation, they underwent, on subsequent days, a screening test with a 50-g oral glucose load and two oral glucose tolerance tests (OGTTs) with 75 and 100 g of glucose according to the WHO and the ADA recommendations, respectively. From this group, we identified 99 women at high risk for GDM, who did not attend their pregnancy follow-up and whose delivery records were recovered at our hospital or in neighbouring hospitals. This unusual situation enabled us to study the natural history and outcome of their pregnancy in spite of not receiving special management usually provided to such women. As macrosomia was expected to be the most frequent undesirable foetal outcome, sensitivity and specificity calculations have been based on this outcome.
Results: The study population (n=99) had a median parity of two and 14% had abnormal results in the 2-h, 75-g load test (WHO) vs. 6% in the 100-g test (ADA). Optimal cut-off points for each test were lower than those recommended for diagnosis by the ADA and the WHO. The optimal sensitivity for the 1-h, 50-g test was 66.7% (cut-off 137 mg/dl), and for the 2-h, 75-g test (cut-off 119 mg/dl). The best specificity and positive predictive value was for this last test with a cut-off point of 140 mg/dl in the second hour.
Conclusions: The standard 2-h cut-off value of 140 mg/dl for the 75-g test, as now recommended by WHO, was optimal for predicting macrosomia. Based on the sensitivity and specificity for macrosomia, the 1-h, 50-g screening test had an optimal cut-off point of 137 mg/dl (vs. 140 mg/dl recommended by ADA). The 2-h, 75-g OGTT value using a cut-off point of 119 mg/dl had equivalent sensitivity, specificity, and positive predictive value. In contrast, the 100-g OGTT had much lower levels of sensitivity, but higher specificity and higher positive predictive value.