Correlations Between Fasting Plasma C-Peptide, Glucagon-Stimulated Plasma C-Peptide, and Urinary C-Peptide in Insulin-Treated Diabetics

This study correlated fasting plasma C-peptide (CP), plasma CP 6 min after stimulation with 1 mg glucagon i.v., and the mean of three 24-h urinary excretions of C-peptide (UCP)/creatinine in 132 insulin-treated diabetics. Patients were divided into three groups: group 1, stimulated CP < 0.06 nM (n = 51); group 2, stimulated CP 0.067−0.60 nM (n = 48); and group 3, stimulated CP > 0.60 nM (n = 33). In all patients fasting CP was closely correlated to stimulated CP (r = .988, P < .001), whereas the correlations between UCP and both fasting CP (r = .904, P < .001) and stimulated CP r = .902, P < .001) were slightly less pronounced. The associations between UCP and both fasting CP (r = .716, P < .001) and stimulated CP (r = .731, P < .001) were modest in group 2, and even more so in group 3 (r = .557, P < .001 and r = .641, P < .001, respectively). In conclusion, fasting CP is closely correlated to glucagon-stimulated CP in insulin-treated diabetics and can probably be used equally well in the assessment of β-cell function. The associations between UCP and both fasting and glucagon-stimulated CP are less pronounced, especially in patients with well-preserved β-cell function.