Residual C-peptide production in type I diabetes mellitus. A comparison of different methods of assessment and influence on glucose control.

The aims of the present study were to compare various methods for assessment of residual insulin production and to evaluate its role in the metabolic regulation in insulin-dependent, type I diabetes mellitus. Glycosylated hemoglobin (HbA1) was used as a measure of the long-term glycemic control. Twenty-eight patients with type I diabetes mellitus with onset before the age of 30 and with a duration of less than 6 years were studied. C-peptide in plasma in the fasting state, after glucagon stimulation, and the 24-hour urinary excretion were measured. Fasting plasma C-peptide was detected in 61%, and 39% showed a significant rise after glucagon stimulation. The increment correlated negatively with HbA1 (rs = -0.57, p less than 0.001), as did the 24-hour urinary excretion (rs = -0.61, p less than 0.001). The 16 patients with urinary C-peptide values of at least 1 nmol had a mean HbA1 of 8.9 +/- 0.3%, as opposed to 11.6 +/- 0.5% for those excreting less (p less than 0.001). Measurement of the 24-hour urinary excretion of C-peptide provides a reliable method for evaluating residual insulin secretion.