Discrimination of type I from insulin-treated type II diabetic patients by C-peptide measurement.

Residual B-cell function was assessed in 61 type I and 17 type II insulin-treated diabetics by measuring plasma C-peptide concentration before and after i.v. injection of 1 mg glucagon to evaluate a possible difference in response to the test in the two groups. Fasting and post-stimulatory C-peptide levels were significantly higher in type II diabetics than in type I (0.45 +/- 0.25 vs 0.12 +/- 0.10 nmol/l for basal IRCP, 0.39 +/- 0.19 vs 0.06 +/- 0.11 nmol/l for delta IRCP, p less than 0.0001), but there was some overlap in individual values. Twenty-one percent of type I and 29% of type II diabetics had values in the overlap area. These percentages were reduced to 6% and 12%, respectively when only long-term (duration of diabetes more than five years) type I diabetics were considered. These data indicate that a glucagon test is useful to discriminate most type I diabetics from insulin-treated type II diabetics.