A comparative assessment of 75g and 100g oral glucose tolerance test in diagnosing gestational diabetes

Background: Gestational Diabetes mellitus (GDM) has potential threatening and undesirable consequences in terms of maternal, foetal, perinatal and neonatal outcomes. Diagnosing GDM could be tricky and confusing given the large number of available criteria. The study was taken up to study the diagnostic characteristics of 75g vs. 100g oral glucose tolerance test (OGTT) read with ADA-2015 and Carpenter Coustan criteria respectively.

Methods: 140 Patients between 24 – 28 weeks gestation with abnormal 50g oral glucose challenge test were randomized to group A and B after excluding pre gestational diabetes. Group A was administered a 75g OGTT and group B was given 100g OGTT. Plasma glucose values were measured at 1st and 2nd hour in group A and 1st, 2nd and 3rd hour in group B.

Results: The 100g OGTT diagnosed 28.57% patients with impaired glucose tolerance (IGT) to have GDM whereas 75g OGTT diagnosed 25.71% of IGT patients to have GDM. The mean plasma glucose levels in mg/dl at 1st and 2nd hour in IGT patients was significantly higher in 100g OGTT (1st hr: 151.82±24.05 vs. 163.29±38.68; P-0.03), (2nd hr: 131.96±27.36 vs 142.7±32.96; P-0.03). Mean plasma in mg/dl in GDM cases varied significantly after 75g and 100g glucose loads (1st hr: 172±23.55 vs 190.75±27.05; P-0.03), (2nd hr: 161.61±14.72 vs 179.35±26.75; P-0.01). 5 and 11 patients met or exceeded abnormal values in 1st hour with 75g OGTT and 100g OGTT (P-0.08) respectively. In the second hour 11 patients showed abnormal values in group A compared to 19 patients in group B (P-0.01). In addition, 9 patients in 100g OGTT group showed abnormal values at 3rd hour.

Conclusion: The 100g OGTT appears to be more sensitive than 75g OGTT. It is preferable to diagnose a patient as GDM and treat her appropriately rather than missing a diagnosis, using a criteria with higher thresholds or lower glucose load. Further studies comparing the outcomes with different diagnostic criteria need to be taken up in quest of one single worldwide criterion applicable and acceptable to most populations.