Fasting C-peptide and Related Parameters Characterizing Insulin Secretory Capacity for Correctly Classifying Diabetes Type and for Predicting Insulin Requirement in Patients with Type 2 Diabetes.

BACKGROUND/AIMS: C-peptide allows estimation of insulin secretion even in the presence of insulin treatment. C-peptide may be suitable for the differential diagnosis of type 1 and type 2 diabetes, and, within type 2 diabetes, of insulin-requiring vs. non-insulin-requiring patients. Relating C-peptide concentrations to ambient glucose levels might improve its diagnostic potential. PATIENTS/
METHODS: The diagnostic value (a) fasting C-peptide, (b) C-peptide/glucose ratios, and (c) the HOMA-ßC-peptide-index for predicting a diagnosis of type 1 (vs. type 2) diabetes were assessed.
SETTING: Specialised hospital for the care of diabetic patients (inpatient treatment). 303 patients with type 1 diabetes and 841 patients with type 2 diabetes. MAIN OUTCOME MEASURE: Odds ratios and 95% confidence intervals for a clinical diagnosis of type 1 diabetes or for insulin treatment by deciles of (a) fasting C-peptide, (b) C-peptide/glucose ratios, and (c) HOMA-ßC-peptide-index.
RESULTS: Low C-peptide concentrations were associated with a high odds ratio for type 1 diabetes and vice versa (p<0.0001). Concentrations of 0.13-0.36 nmol/l did not discriminate. C-peptide/glucose ratios or HOMA-ßC-Peptide did not perform better. The ability of all 3 parameters to predict the necessity for insulin treatment within the population of type 2-diabetic patients was low.
CONCLUSIONS: Fasting C-peptide and derived parameters help to differentiate type 1 from type 2 diabetes, but there is a range of C-peptide concentrations that does not help discriminate. Relating C-peptide to glucose did not improve diagnostic accuracy. C-peptide does not help predicting a need for insulin treatment in patients with type 2 diabetes. © Georg Thieme Verlag KG Stuttgart · New York.