Retnakaran R, et al. - In this investigation, researchers determined the stability of insulin and C-peptide concentrations that were first measured on fresh serum at the time of collection, followed by frozen storage at -80oC for >5 years and then repeat measurement. Bland-Altman analyses showed that there was good agreement between repeated insulin measurements and repeated C-peptide measurements. For both insulin and C-peptide, the Concordance Correlation Coefficient (CCC) confirmed reproducibility. Therefore, insulin and C-peptide measurements are both stable and reproducible after long-term frozen serum samples storage.