Evaluation of B-cell function in diabetics by C-peptide determination in basal and postprandial urine

Urinary C-peptide (UCP) values in basal and postprandial 2-hour samples are compared with simultaneous plasma C-peptide (PCP) values and with 24-hour UCP in 21 NIDDs, 24 IDDs and 9 controls. Basal and postprandial 2-h UCP was similar in controls and NIDDs (0.79 +/- 0.53 and 2.68 +/- 1.06 nmol/2 h versus 1.03 +/- 0.70 and 2.64 +/- 1.34, p greater than 0.05) and very low in IDDs (0.16 +/- 0.22 and 0.58 +/- 0.86, p less than 0.001). NIDDs showed basal (0.69 +/- 0.32 nmol/l) and postprandial (1.12 +/- 0.50) PCP values lower than controls (0.91 +/- 0.31 and 1.90 +/- 0.70). UCP clearance was higher in NIDDs than in controls. IDDs showed similar pattern of higher clearance but greater variability, the differences not being significant. It is concluded that despite correlations between PCP and UCP, possible differences can exist in C-peptide renal handling in diabetics, making unwise to use UCP as sole parameter of B-cell function in these patients.