Effect of temperature and preservative selection on C-peptide degradation in randomly voided urine

• Teaching Points: Urine C-peptide (UCP), a noninvasive biomarker of β cell function, is susceptible to degradation that may be dependent upon temperature, preservative, and sample handling conditions. • Sodium carbonate effectively preserves UCP:creatinine ratio (UCP:Cr) for up to 72 h, particularly at 4°C, outperforming boric acid and no preservative conditions. • A hybrid storage method using initial refrigeration followed by cold-pack transport maintains UCP stability equivalent to continuous refrigeration. • Omission of centrifugation markedly accelerates UCP degradation, underscoring its importance as a pre-analytical processing step. • These findings provide practical guidance for remote urine collection and shipment protocols to improve biomarker reliability in type 1 diabetes clinical trials. • Clinical Relevance: Optimizing pre-analytical handling of urine for C-peptide measurement improves the reliability of noninvasive assessments of β cell function and enables decentralized sample collection in type 1 diabetes clinical studies. Type 1 diabetes (T1D) results in the loss of endogenous production of insulin and C-peptide, which are co-secreted by pancreatic β cells. Serum C-peptide following mixed-meal tolerance tests is commonly used in T1D trials to assess residual insulin secretion. Urine C-peptide (UCP) offers a non-invasive alternative suitable for home collection. Here, we aimed to optimize urine handling practices to support remote urine C-peptide monitoring in clinical trial settings. Randomly-voided urine was collected from 18 non-diabetic participants (mean age: 46.6 years; 94.4% female). Samples were treated with no preservative (NP), boric acid (BA), or sodium carbonate (SC) and incubated at 4°C or room temperature (RT, 22°C) for up to 72 h. Use of cold packs after initial refrigeration was also tested. UCP was quantified using the TOSOH immunoenzymatic assay and normalized to urine creatinine (UCP:Cr). Stability was expressed as percent change from baseline (%UCP:Cr). NP samples lost %UCP:Cr more rapidly at RT, with decreases at 48 h (−16%, P < 0.05) and 72 h (−21%, P = 0.02) compared to 4°C. BA-preserved samples had declined %UCP:Cr at 72 h (−16%; P = 0.04) compared to 4°C. SC maintained UCP:Cr stability, showing no significant change in %UCP:Cr between 4°C and RT. Samples kept on cold packs following initial refrigeration showed no significant loss in %UCP:Cr compared to samples continuously refrigerated for 72 h. Cold storage of urine slows UCP:Cr degradation, and SC stabilizes UCP:Cr at room temperature. Our findings suggest that UCP:Cr offers a practical option for at-home urine collection in clinical studies