GLUCOSE-TOLERANCE TEST PERIODICITY - THE EFFECT OF GLUCOSE LOADING

Objective: To test the hypothesis that glucose abnormality, as shown b y glucose tolerance test (GTT) periodicity, is not affected by differe nt glucose loads, allowing for the identification of gestational diabe tes mellitus (GDM) under varying glucose challenges. Methods: Eighty s ubjects were tested by multiple GTTs 1 week apart. Each woman served a s her own control, undergoing a standard 3-hour, 100-g GTT; then, half of the subject group randomly underwent a 50-g and the other half a 7 5-g, 2-hour GTT. Subjects were classified using National Diabetes Data Group thresholds for the 100-g GTT. Those with two or more abnormal v alues were classified as gestational diabetic (GDM group); the rest of the women were considered to be nondiabetic. The projected time for t he GTT to revert to fasting value, GTT periodicity, was then determine d for each glucose load in the GDM and nondiabetic groups. Results: Al l glucose values for the nondiabetic group were significantly lower at 1 and 2 hours than those for the GDM group, regardless of the glucose load (P < .04). There was a statistically significant difference with in the GDM and nondiabetic groups between glucose values of the 100- a nd 50-g GTTs at 1 hour (P < .02) and between all loads at 2 hours (P < .04). The GTT periodicity for the 3-hour, 100-g test was significantl y longer for patients with GDM, as shown previously (5.6 +/- 1.9 versu s 3.2 +/- 1.7 hours, P < .0001). In addition, similar values were foun d for nondiabetic and GDM subjects for the 75-g (5.1 +/- 2 versus 3.6 +/- 1.8 hours, P < .04), but not the 50-g load (2.2 +/- .6 versus 1.34 +/- .8 hours, P < .01). Conclusion: Glucose tolerance test periodicit y will identify subjects with GDM regardless of GTT load because the p hysiologic disturbance of glucose level measured by this time period r emains comparably longer than in normal subjects. We speculate that th e relatively shorter cycle of the 50-g load may reflect an insufficien t challenge to pancreatic function.