Plasma and urinary C-peptide in the classification of adult diabetics.

Plasma and urinary C-peptide determinations in the discrimination between insulin-requiring and non-insulin-requiring diabetes were elevated in 61 adult diabetics. Specimens for C-peptide determinations were taken on two consecutive days: on the first day plasma C-peptide concentrations were determined before and 6 min after intravenous glucagon administration. On the second day 2- and 4-h urinary C-peptide excretion was measured after an individual breakfast. Results of urinary C-peptide analyses were expressed as molar concentration and also as molar quantity excreted (without any corrections and related to creatinine excretion). Glucagon-stimulated plasma C-peptide turned out to be a reliable criterion for the detection of insulin requirement. Sixty-nine per cent of diabetics included in this study were classifiable by basal plasma C-peptide concentrations. Two-hour postprandial urinary C-peptide/creatinine quotient turned out to be slightly less sensitive (89%) than the glucagon test (94%) and of equal specificity (96%). Glucagon-stimulated plasma C-peptide and postprandial urinary C-peptide excretion correlated significantly among insulin-requiring diabetics (r = 0.73), but not among non-insulin-requiring diabetics (r = 0.23). We regard determination of stimulated plasma C-peptide as a primary investigation for the direct assessment of endogenous insulin secretory reserves for clinical management decisions. Determination of postprandial urinary C-peptide is applicable in selected situations for non-invasive assessment of insulin secretion.