Association between area under the C-peptide curve during an oral glucose tolerance test and diabetic retinopathy in type 2 diabetes patients

Aims/introduction: To evaluate the relative contributions of the area under the C-peptide curve (AUC(C) ) in diabetic retinopathy (DR) during an oral glucose tolerance test and C-peptide release test in patients with type 2 diabetes.Materials and methods: We retrospectively analyzed the data of 969 patients. Their general characteristics were retrieved. A series of parameters for assessing pancreatic beta-cells function, such as the AUC(C) for six time periods: 0-60 min (AUC(C0-60) ), 0-120 min (AUC(C0-120) ), 0-180 min (AUC(C0-180) ), 60-120 min (AUC(C60-120) ), 60-180 min (AUC(C60-180) ) and 120-180 min (AUC(C120-180) ); the area under the glucose-time curve for six time periods: 0-60 min (AUC(G0-60) ), 0-120 min (AUC(G0-120) ), 0-180 min (AUC(G0-180) ), 60-120 min (AUC(G60-120) ), 60-180 min (AUC(G60-180) ) and 120-180 min (AUC(G120-180) ) and their related indexes, were calculated through 0-180 min oral glucose tolerance test and C-peptide release test. We used univariate analysis to examine the potential factors affecting DR. Spearman's correlation was used to analyze the correlation between AUC(C) -related indexes and DR. The logistic regression model was used to investigate AUC(C) and its related indexes' contribution to incidence DR. A smooth curve fitting model was used to determine the correlation, non-linear relationship, and threshold effect between AUC(C) and DR.Results: Of the 969 patients with type 2 diabetes, 469 (48.40%) and 500 (51.60%) were classified as the DR group and non-DR group. Compared with the non-DR group, the DR patients had lower AUC(C) and AUC(C) /AUC(G) . Spearman's correlation analysis showed that AUC(C) -related indexes were all negatively correlated with DR. The logistic regression analysis determined that there were associations between AUC(C) and DR in the adjusted models. The odds ratio values of AUC(C0-60) , AUC(C0-120) , AUC(C0-180) , AUC(C0-60) /AUC(G0-60) , AUC(C0-120) /AUC(G0-120) , AUC(C0-180) /AUC(G0-180) , AUC(C60-120) , AUC(C60-180) , AUC(C120-180) , AUC(C60-120) /AUC(G60-120) , AUC(C60-180) /AUC(G60-180) and AUC(C120-180) /AUC(G120-180) were 0.817 (0.750, 0.890), 0.925 (0.895, 0.955), 0.951 (0.932, 0.970), 0.143 (0.060, 0.340), 0.194 (0.093, 0.406), 0.223 (0.116, 0.427), 0.886 (0.842, 0.933), 0.939 (0.915, 0.963), 0.887 (0.846, 0.930), 0.253 (0.133, 0.479), 0.282 (0.160, 0.497) and 0.355 (0.220, 0.573), respectively. AUC(C) showed a non-linear relationship with DR, with an inflection point. The inflection points of AUC(C180) /AUC(G180) , AUC(C60-120) , AUC(C60-180) , AUC(C120-180) , AUC(C60-120) /AUC(G60-120) , AUC(C60-180) /AUC(G60-180) , AUC(C120-180) /AUC(G120-180) and DR were 17.51, 0.542, 6.6, 15.7, 8.23, 0.534, 0.593 and 0.808 (P < 0.0001). When the indexes related to the AUC(C) were less than the inflection point value, they were significantly negatively associated with DR.Conclusions: The indexes related to the AUC(C) for six time periods during an oral glucose tolerance test and C-peptide release test was closely associated with the incidence to DR in patients with type 2 diabetes. AUC(C) has the added advantage of being a cheap and convenient risk assessment over traditional ophthalmic screening.