The influence of glucose tolerance tests on subsequent carbohydrate metabolism in pregnancy

Background. This present study was designed to prospectively confirm a previous observational finding that a 75g glucose load exerts an influence on the results of a subsequent 50g glucose tolerance test performed after an interval of one week. Methods. Pregnant Chinese patients were given both 50g 3-hour and 75g 2-hour oral glucose tolerance rest (OGTT) in random order, 7 days apart, between 28 and 32 weeks of gestation. Patients were divided according to whether they received 50g test first (group 1) or 75g test first (group 2). Student's t-test for independent samples and for paired samples were used for between group comparisons and within patient comparisons respectively using SPSS for Windows software. Results. Significant decrease in fasting glucose levels (p<0.0001) between paired 50g and 75g OGTT was found when 75g test was performed first. The mean difference in 2-hour values was also 35% lower. Similar results were also found when individual timed glucose levels were compared between groups. Conclusions. Differences in the blood glucose levels between OGTTs were confirmed to be related to which glucose load was given first. The 75g test exerts a significant effect on the fasting glucose and the response to a 50g glucose load up to one week later. These effects are consistent with improvement in glucose utilization. The differences observed will not significantly alter current clinical practice but may necessitate modification in the diagnostic criteria of mild abnormal glucose tolerance in pregnancy. We postulate that in some cases of mild glucose intolerance, the response to diet may not be due to caloric reduction but rather stimulation of a previously inadequate insulin response since a proportion of our local pregnant women consistently have caloric intakes significantly below the 1800 kcal per day.