Stability of insulin and C-peptide measurement with long-term frozen storage of serum: Implications for diabetes research studies.

The current era of large-scale clinical trials in diabetes has generated thousands of biological samples from study participants that are being stored long-term under frozen conditions for the future measurement of analytes of interest. Insulin and C-peptide are two such analytes that can provide insight into underlying pathophysiological processes (insulin sensitivity and β-cell function). However, the validity of the inferences that may be drawn from such future measurements is contingent on the stability of these analytes after long-term frozen storage. We conducted the present study to determine 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 -80°C for >5 years and then repeat measurement. Bland-Altman analyses revealed good agreement between the repeated insulin measurements and between the repeated C-peptide measurements. The concordance correlation coefficient (CCC) confirmed reproducibility for both insulin (CCC 0.98, 95% confidence interval [CI] 0.96-0.99) and C-peptide (CCC 0.91, 95% CI 0.84-0.95); thus, insulin and C-peptide measurements are both stable and reproducible after long-term frozen storage of serum samples.