Stability of Nε-Carboxymethyllysine and Nε-Carboxyethyllysine in Canine Urine Under Extended Room Temperature Storage

Advanced glycation end products (AGEs) such as Nε-carboxymethyllysine (CML) and Nε-carboxyethyllysine (CEL) are implicated in chronic disease processes in humans and may serve as biomarkers of dietary exposure and metabolic health. Urinary measurement of AGEs is of interest due to its non-invasive nature and relevance to biobanking and field-based sample collection; however, AGE stability in urine under common handling conditions has been poorly characterized. This study evaluates the short-term stability of CML and CEL in canine urine stored at room temperature (20 °C) for up to 168 h prior to -80 °C storage. Midstream free-catch urine samples from eight healthy dogs were aliquoted, stored at defined intervals, and analyzed in duplicate using liquid chromatography-mass spectrometry (LC-MS) with isotope-labeled standards. Results demonstrate minimal detectable changes in CML and CEL concentrations, as well as in the CML/CEL ratio, over the ambient storage period. Inter-replicate agreement is high, and regression and non-parametric analyses show no association between storage duration and analyte concentration. These findings indicate that urinary CML and CEL measurements may remain reliable despite delayed processing, supporting field-based sampling and retrospective analyses. Evaluation of additional AGE species and storage conditions will further inform best practices for sample handling in veterinary and comparative biomedical research.