Glucose tolerance test periodicity: the effect of glucose loading

To test the hypothesis that glucose abnormality, as shown by glucose tolerance test (GTT) periodicity, is not affected by different glucose loads, allowing for the identification of gestational diabetes mellitus (GDM) under varying glucose challenges.

Eighty subjects were tested by multiple GTTs 1 week apart. Each woman served as 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 75-g, 2-hour GTT. Subjects were classified using National Diabetes Data Group thresholds for the 100-g GTT. Those with two or more abnormal values were classified as gestational diabetic (GDM group); the rest of the women were considered to be nondiabetic. The projected time for the GTT to revert to fasting value, GTT periodicity, was then determined for each glucose load in the GDM and nondiabetic groups.

All 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 within the GDM and nondiabetic groups between glucose values of the 100- and 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 significantly longer for patients with GDM, as shown previously (5.6 +/- 1.9 versus 3.2 +/- 1.7 hours, P < .0001). In addition, similar values were found 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).

Glucose tolerance test periodicity will identify subjects with GDM regardless of GTT load because the physiologic disturbance of glucose level measured by this time period remains comparably longer than in normal subjects. We speculate that the relatively shorter cycle of the 50-g load may reflect an insufficient challenge to pancreatic function.