Serum free C-peptide response to oral glucose loading as a parameter for the monitoring of pancreatic B-cell function in diabetic patients

As a parameter for evaluating B-cell function, the accuracy of measuring serum free C-peptide immunoreactivity (CPR) was compared with that of measuring plasma immunoreactive insulin (IRI) and urine CPR in diabetic patients during a 100 g oral glucose tolerance test. In 25 non-obese patients receiving oral hypoglycemic agent or diet treatment alone, a positive correlation between the sum of serum free CPR (Σ serum free CPR) and the sum of plasma IRI (Σ plasma IRI) was noted (r = 0.68, P < 0.001). However, the sum of blood glucose values was found to be negatively correlated to Σ serum free CPR (r = -0.56, P < 0.0025), but not to Σ plasma IRI (r = -0.25, NS). In 23 patients receiving diet, oral hypoglycemic agent or insulin treatment, a positive correlation between Σ serum free CPR and urine CPR was noted (r = 0.75, P < 0.001). However, no significant correlation was found when only insulin-treated patients were investigated (r = 0.37, NS, n = 17). In addition, patients with insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus were better differentiated by measuring Σ serum free CPR than urine CPR. Thus, we concluded that the measurement of serum free CPR during OGTT provides an extremely valuable method for monitoring pancreatic B-cell function in diabetic patients, whether they are receiving insulin treatment or not.