Serum C-peptide concentrations and their value in evaluating the usefulness of insulin therapy in elderly diabetics.

Serum C-peptide concentrations were determined in 121 elderly subjects: 25 nondiabetic controls aged 69-86 years, and 96 type 2 (noninsulin-dependent diabetes mellitus) diabetics aged 64-96 years. Forty-seven of the diabetics were treated with tablets, 35 with insulin, and 14 with diet alone. Fasting serum C-peptide concentrations (nmol/l; mean +/- SD) were 0.51 +/- 0.20 for controls; 0.60 +/- 0.16 for diabetics on diet alone; 0.72 +/- 0.33 for diabetics on tablets and 0.46 +/- 0.23 for diabetics on insulin (p less than 0.001 for diabetics on tablets vs. controls and diabetics on tablets vs. diabetics on insulin). The glucagon-stimulated C-peptide concentrations were similar in all groups; the increment after glucagon was less in the diabetic patients on tablets or on insulin than in the nondiabetics. In 10 patients on insulin treatment and with fasting C-peptide of 0.24-1.46 nmol/l an attempt was made to withdraw insulin. In 4 subjects the transfer to tablets was possible. Serum C-peptide level did not predict the outcome of the attempt to change the therapy, but the possibility of an adequate dietary regimen seemed to be important. The results demonstrate a wide range of basal C-peptide concentrations in elderly diabetics on different treatments, which may indicate varying pathogenetic contributions of insulin deficiency and resistance in these patients. Our observations emphasize the necessity for regular re-evaluation of the therapeutic management of elderly diabetic patients.