Urinary biomarkers of kidney transplant rejection.

Purpose of review: Despite the introduction of many new immunosuppressive medications, allograft rejection remains a significant complication in transplantation. The use of "liquid biopsy" to evaluate allograft function and detect early rejection has recently become a prominent focus of investigation as it holds promise in providing noninvasive and immediate insights into the cellular and molecular makeup of the graft.

Recent findings: In recent years, the introduction of molecular medicine along with the use of new technologies, including high-throughput techniques, has not only accelerated biomarker discovery but has also contributed to improving our understanding of the mechanisms underlying immune rejection. Genomics, transcriptomics, and metabolomics approaches, along with the increasing use of machine learning techniques, have paved the way for the discovery and development of novel biomarkers.

Summary: Each year, there are hundreds of new biomarker discoveries in the publications. However, only a small fraction can be practically used as clinical tests or surrogate endpoints, receive FDA approval, and reach clinical application. Well designed and reproducible discovery and validation studies are rare and crucial. A contributing factor could be poor study design or quality of biospecimen repositories. In this review, we discuss urinary biomarkers of kidney allograft rejection that have shown promising findings but have yet to be successfully transitioned from bench to bedside.