The glucose tolerance test, also known as the oral glucose tolerance test, measures body's response to sugar (glucose). The glucose tolerance test can be used to screen for type 2 diabetes. More commonly, a modified version of the glucose tolerance test is used to diagnose gestational diabetes - a type of diabetes that develops during pregnancy.
Identifying women with gestational diabetes mellitus (GDM) allows interventions to improve perinatal outcomes. A fasting plasma glucose (FPG) level ≥5.1 mmol/L is 100% specific for a diagnosis of GDM.

Gestational diabetes mellitus (GDM) is a frequent medical complication during pregnancy. Screening and diagnostic practices for GDM are inconsistent across the world. Screening in early pregnancy remains controversial and studies show conflicting results on the benefit of screening and treatment of GDM in early pregnancy. Implementing the one-step "International Association of Diabetes and Pregnancy Study Groups" (IADPSG) screening strategy at 24-28 weeks often leads to a substantial increase in the prevalence of GDM, without conclusive evidence regarding the benefits on pregnancy outcomes compared to a two-step screening strategy with a glucose challenge test (GCT). In addition, RCTs are needed to investigate the impact of treatment of GDM diagnosed with IADPSG criteria on long-term maternal and childhood outcomes. Selective screening using a risk-factor-based approach could be helpful in simplifying the screening algorithm but carries the risk of missing significant proportions of GDM cases.
To assess the prevalence and association of frequently used screening risk factors for gestational diabetes mellitus (GDM) and to compare the validity and cost of universal screening with risk factor screening, Shamsuddin, K and team carried out a cross-sectional survey of 768 pregnant women at ≥24 weeks’ gestation who were attending the antenatal clinic at the Hospital Universiti Kebangsaan Malaysia (HUKM) was made. Risk factors were determined using a questionnaire. An abnormal oral glucose tolerance test was defined as a 2-h post-prandial blood sugar level of ≥7.8 mmol/l.

A total of 191 pregnant mothers (24.9%) had GDM. The most commonly identified screening factors were positive family history of diabetes mellitus (31.4%), history of spontaneous abortion (17.8%), vaginal discharge and pruritis vulvae in current pregnancy (16.0%), and maternal age greater than 35 years (14.7%). Five hundred and thirteen mothers (66.8%) had at least one risk factor.
All screening risk factors, except past history of diabetes mellitus in previous pregnancy and maternal age, were not significantly associated with abnormal glucose tolerance (GT). Risk factor screening gave a sensitivity of 72.2% and a specificity of 35.0%. Universal screening would cost RM 12.06 while traditional risk factor screening would cost RM 11.15 per identified case and will have missed 53 of the 191 cases.
Risk factor screening scored poorly in predicting GDM. Cost analysis of universal compared with traditional risk factor screening showed a negligible difference. Thus universal screening appears to be the most reliable method of diagnosing GDM.
Source: Shamsuddin, K., Mahdy, Z.A., Siti Rafiaah, I., Jamil, M.A. and Rahimah, M.D. (2001), Risk factor screening for abnormal glucose tolerance in pregnancy. International Journal of Gynecology & Obstetrics, 75: 27-32. https://doi.org/10.1016/S0020-7292(01)00468-4