Figure 1. C-peptide loss follows exponential decay.

(A) C-peptide AUC from 2-hour mixed-meal tolerance test. Each line shows an individual subject’s values across multiple visits. The dashed line represents the lower limit of detection of the assay. n = 846 measurements from 152 subjects. (B) Prediction of C-peptide AUC at 2 years based on baseline C-peptide AUC, age at study entry, and AUC at 6- or 12-month visits. Predicted values are model predictions from leave-one-out cross-validation. Predictive R² summarizes correspondence between observed and predicted values; the dashed line represents equivalence of predicted and observed values. n = 109 subjects for each model. (C) Rate of C-peptide AUC change varies with age. Model fit line is based on a logarithmic function; shading shows standard error of the model. Variance in C-peptide change is greater in younger subjects (Breusch-Pagan test, P = 0.002). n = 152 subjects. (D) Rate of C-peptide change does not vary consistently with HLA genotypes that confer T1D risk. The dashed line shows linear model fit (P = 0.4). Genotype categories are from Winkler et al. (28); DRx represents alleles that are not DR3 or DR4-DQ8. n = 124 subjects with HLA genotyping.