Glucose tolerance test periodicity as a descriptor of glucose tolerance abnormality.

The periodicity of the standard 100-g glucose tolerance test (GTT) was examined in a prospective study of 194 pregnant patients to determine how well gestational diabetes could be identified. A simplified formula, the GTT periodicity, was used to estimate the time for the GTT curve to return to the fasting level. One hundred one study subjects had all normal glucose values by the National Diabetes Data Group criteria (0-abnormal group), 47 had one value greater than normal (1-abnormal group), and 46 had more than one value abnormal or gestational diabetes. The 0-abnormal patients had a significantly shorter GTT periodicity than did 1-abnormal or gestational diabetic mothers (3.6 versus 4.8 versus 6.6 hours, respectively; P less than .04). Calculating the periodicity for the corresponding insulin excursions yielded significantly increasing values in a graduated fashion for each group (5.2 versus 6.9 versus 9.6 hours, respectively; P less than .05). Examination of the oscillation of the GTT curve about the fasting level allows a physiologic description of normal and abnormal glucose responses in pregnancy. Furthermore, our findings suggest that glucose and insulin periodicities are useful predictors of gestational diabetes in patients with positive screening.