An elevated glucose tolerance curve is widely accepted as indicative of the presence of diabetes mellitus.1,2 Caution has been advised, however, in assuming that an elevated response to a glucose load is, in itself, proof of the existence of this disease in patients with fasting normoglycemia.3,4 Uncertainty has arisen primarily because the oral tolerance test lacks specificity. Almost always abnormal in patients with diabetes, the test is frequently abnormal in nondiabetics as well. Thus, the incidence of a "diabetic" response to oral glucose in a random population sample was 15 times the estimated frequency of diabetes in the general population.5 Only between 20% and 30% of subjects with normal fasting levels but elevated curves developed overt diabetes or fasting hyperglycemia during 5- to 30-year follow-up studies.6-9 Just as many reverted to normal as became diabetic.9
The diagnostic unreliability of the oral test has been attributed,
KAPLAN NM. Tolbutamide Tolerance Test in Carbohydrate Metabolism Evaluation. Arch Intern Med. 1961;107(2):212–224. doi:10.1001/archinte.1961.03620020062006
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