C-peptide measurement in the differentiation of type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes mellitus.

To determine whether individual subjects with Type 1 (insulin-dependent) diabetes or Type 2 (non-insulin-dependent) diabetes, who are treated with insulin, could be reliably distinguished, C-peptide concentrations and urinary C-peptide excretion were measured in 10 Caucasoids and 10 Pima Indians. All the subjects had developed diabetes before 21 years of age and were receiving insulin treatment. Fasting C-peptide concentrations were significantly higher in the Pima Indians (0.73 +/- 0.17 versus 0.02 +/- 0.01 nmol/l in Caucasoids; p less than 0.001), but there were slight overlaps in individual values. Urinary C-peptide excretion, an index of 24-h-insulin excretion, was also higher in the Pima Indian group (27.6 +/- 1.85 versus 0.72 +/- 0.18 pmol/min in Caucasoids; p less than 0.001) and there was no overlap in the individual values between the groups. The Pima Indians with early onset diabetes have been previously shown to have Type 2 diabetes, and the Caucasoids with an early onset are most likely to have Type 1 diabetes. These results suggest that distinction between these two major types of diabetes can be made effectively by using C-peptide measurements provided that overt renal disease is absent. This differentiation between insulin-treated patients will be useful for a variety of research applications and possibly in making clinical management decisions.