In a prospective study of 41 consecutively referred newly diagnosed diabetic patients, we evaluated the predictive value of fasting and glucagon-stimulated C-peptide values, ketonuria, age, and body weight in the classification of subjects as insulin-requiring (IR) or non-insulin-requiring (NIR). The patients were followed up for ≥12 mo and classified as NIR if adequate glycemic control could be achieved without insulin (i.e., fasting plasma glucose < 8 mM and no glycosuria). Patients who needed insulin to obtain this status were classified as IR. We found that all subjects with plasma C-peptide values >0.60 nM 6 min after intravenous glucagon were NIR, whereas all IR subjects together with 3 NIR subjects had C-peptide values below this limit. All NIR subjects but 1 had fasting C-peptide values > 0.30 nM, and all IR subjects but 1 had C-peptide values below this limit. Seventy-five percent of the subjects could be correctly classified by use of age and percent desirable body weight. Thus, all subjects > 40 yr old and >100% ideal body weight were NIR, and all subjects below both these limits were IR. Ketonuria was found in 10 of 12 IR subjects and in 10 of 29 NIR subjects. We conclude that 1) 75% of the subjects could be correctly classified by use of age and percent desirable body weight only and 2) C-peptide measurements are useful in the classification of newly diagnosed diabetes, whereas presence of ketonuria is of limited value.

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