Saliva as a diagnostic matrix for renal function

Assessment of renal function in clinical practice is largely dependent on serum-based biomarkers, particularly urea and creatinine, which remain central to the diagnosis and monitoring of kidney disease. Although analytically robust, blood-based testing requires venipuncture and laboratory infrastructure, limiting its practicality for frequent monitoring, large-scale screening, and use in vulnerable or resource-constrained populations. In this context, saliva has gained attention as a non-invasive biological matrix with potential diagnostic relevance. Salivary urea and creatinine are detectable in measurable concentrations and, in patients with renal impairment, often show positive associations with corresponding serum levels. These observations suggest that saliva may reflect systemic alterations in nitrogenous waste handling, particularly in moderate-to-advanced renal dysfunction. However, the clinical translation of salivary renal biomarkers is constrained by biological variability, low analyte concentrations, matrix-specific interferences, and the absence of standardized collection and analytical protocols. This review critically examines the biochemical basis for the appearance of urea and creatinine in saliva, evaluates current analytical methodologies and validation strategies, and summarizes evidence from comparative serum-saliva studies. The potential clinical applications of saliva-based testing, along with its limitations and future directions for integration into laboratory medicine, are discussed with an emphasis on translational feasibility rather than replacement of established serum assays.