Basal serum C-peptide concentrations in diabetic patients showed two groups. Diabetic patients with low C-peptide levels (≤ 0.16 nmol/L) have clinical characteristics of type I diabetes, and all were on insulin therapy. With long duration of diabetes, an increasing proportion had undetectable C-peptide. Diabetic patients with high C-peptide levels (< 0.16 nmol/L) resemble type II diabetes. In this group 30% were on insulin therapy but duration of known disease was not associated with any decline in the high basal C-peptide levels. The small proportion of diabetic patients with basal serum C-peptide in the range of 0.17–0.32 nmol/L have indeterminate status.

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