C-peptide reactivity as a measure of insulin dependency in obese diabetic patients treated with insulin.

The C-peptide increase after an oral glucose tolerance test (OGTT) and intravenous glucagon test (GT) was assessed to establish actual insulin dependency in 16 obese diabetic patients treated with insulin for years. The results, compared to five reference groups, showed two types of reaction: first, a negative response in 3 patients with a minimal C-peptide increase after either stimulation (range 0-0.1 ng/ml during GT, 0-0.5 during OGTT), not different from that in the insulin-dependent reference group (GT: 0.1 +/- 0.2, OGTT: 0.1 +/- 0.3. means +/- SD). Second, a positive response in 13 patients with a C-peptide increase, different from that in the insulin-dependent group after glucagon (range 0.7-5.6), and in 6 patients after OGTT (range 0.8-1.8). When insulin treatment was discontinued in two nonresponders, the B-hydroxybutyrate levels and ketobutyrate levels increased to over 2.1 and 0.60 mmol/L within 10 days, proving insulin dependency, and thus type I diabetes. B-hydroxybutyrate and ketobutyrate levels increased to maximal 0.4 and 0.17 after stopping insulin therapy in nine positive responders. No ketoacidosis developed during a 4-wk follow-up, indicating non-insulin dependency (type II diabetes). Measuring C-peptide after glucagon is a simple test that may be a discriminative method to establish insulin dependency in obese diabetic patients treated with insulin.