MON-623 Pattern of C-Peptide Response to Oral Glucose Tolerance Test: Interest and Cut-Off Values

Abstract

Introduction: Oral glucose tolerance test (OGTT) allows classification of subjects in 3 groups, depending on glycaemia 120 minutes after 75g glucose ingestion: normal (glycaemia < 1.4 g/L), glucose intolerant (1.4-2 g/L) and diabetic (>2g/L). Five insulin profiles following OGTT associated with different incidence rates of diabetes over 10 years of follow-up have also previously been described (Kraft J et al, Laboratory Medicine, 1975; Hayashi T et al, Diabetes Care.2013). Insulin measurement is very sensible to hemolysis and can advantageously be replaced by C-peptide determination. However, little is known about C-peptide reference values and response to OGTT.Material and Methods: 128 patients were included to evaluate glyceamia (COBASe801® ROCHE Diagnostics, France), insulin and C-peptide (LiaisonXL®, Diasorin, France) responses to OGTT.Results: According to Hayashi classification, 23 (18%) patients of the whole cohort harbored a physiological insulin response corresponding to profile I (peak of insulin during OGTT at 30 min and higher insulin level at 60 vs. 120 min). Others presented 5 pathological profiles: 14 (11%) patients were classified in profile II (peak of insulin at 30 min and lower or equal insulin level at 60 vs. 120 min), 56 (44%) in profile III (peak of insulin at 60 min), 26 (20%) in profile IV (peak of insulin at 120 min and lower insulin level at 30 vs. 60 min), and finally 9 (7%) in profile V (peak of insulin at 120 min and higher or equal insulin level at 30 vs. 60 min). Only 4 different mean C-peptide profiles emerged from the subgroups previously defined by insulin profile, mean C-peptide profile being substantially similar to mean insulin profile. The only major difference relied on a similar C-peptide profile corresponding to a growing curve from T0 to T120 in both patients with insulin profile IV and V. Mean and 95% confidence interval of C-peptide value at the different times of OGTT were also calculated in the subgroup of patients with both normal glycemic and insulin (pattern I) responses to propose reference values: respectively T0: 0.53 (0.26-0.77); T30: 2.2 (1.24-3.29); T60: 2.26 (1.36-3.68); T120: 1.88 (0.84-2.62) nmol/L. Conclusion: C-peptide response to OGTT profile seems to give globally the same information as insulin profile and should therefore also be predictive of the risk type 2 diabetes in case of hemolyzed samples. The slight differences observed between insulin and C-peptide profiles can be explained by their different metabolic pathways, insulin being quickly degraded in the liver and C-peptide undergoing a longer renal elimination. This work also allows us to propose for the first-time reference values for C-peptide at the different times of OGTT using Liaison XL®.