The size of the loading dose as an important determinant of the results of the oral glucose tolerance test: a study in subjects with slightly impaired glucose tolerance.

In the oral glucose tolerance test (OGTT), divergent doses of glucose remain in use by virtue of the prevailing conviction that the size of the loading dose hardly affects the outcome of the test. We compared the results of OGTTs with 100-gm. and 50-gm. loads in 85 patients, who were selected for slightly impaired glucose tolerance (plasma glucose at 120 minutes after 100 gm. of glucose was between 130 and 200 mg./dl.) The mean between-load difference in this group appeared to be nearly three times as great (54 mg./dl. at 120 minutes) as reported in the literature for normal subjects. The small impact of the dose in normal subjects could be confirmed in a group of 22 controls. As subjects with normal and with slightly impaired glucose tolerance react divergently to a change in the glucose dose, tests with different loads are not comparable and select different populations. The results can therefore also not be converted to one another by conversion formulas. The finding might be explained by the delay of the additional rise of the plasma insulin in patients after the higher load.