Objectives: Replacement of the two-step, 100 gm, 3-hour National Diabetes Data Group procedure by the one-step, 75 gm, 2-hour World Health Organization oral glucose tolerance test has been hindered by a paucity of data comparing the two tests during pregnancy. The current series compared 100 gm and 75 gm glucose loads and glucose measurements in venous plasma or capillary blood.
Study design: After a 75 gm oral glucose tolerance test 30 gestational diabetics and 30 metabolically healthy pregnant women were randomly assigned to a second 75 or 100 gm test within 3+/-1.3 (mean+/-SD) days. Glucose levels at both tests was measured in capillary blood and venous plasma, as were insulin and C peptide.
Results: In controls 1-hour maternal glucose levels (112 vs 128 mg/dl) and 2-hour levels (104 vs 113 mg/dl) differed significantly after a 75 or 100 gm load (paired t test). In gestational diabetes mellitus, however, there was no difference (176 vs 178 mg/dl) but a low insulin/glucose quotient at 1 hour. Only 2-hour levels differed significantly (133 vs 149 mg/dl). In controls glucose measurement in capillary blood and venous plasma differed significantly at 1 hour (126 vs 115 mg/dl) and 2 hours (111 vs 104 mg/dl) independently of the glucose load. In gestational diabetes mellitus, however, glucose measurement in capillary blood and venous plasma differed neither in 1-hour levels (179 vs 174 mg/dl) nor in 2-hour levels (142 vs 139 mg/dl).
Conclusion: In metabolically healthy women both different loading and different blood fractions lead to statistically different blood glucose levels at 1 and 2 hours. In gestational diabetes mellitus, however, 1-hour glucose levels do not differ after a 75 or 100 gm load or after glucose measurement in capillary blood or venous plasma. This is due to elevated insulin resistance shown by a low insulin/glucose quotient at 1 hour. For comparison of tests in gestational diabetes mellitus only, 2-hour values must be adjusted by 16 mg/dl after different loading.