What new renal biomarkers tell us about renal physiology: Collection: physiology applied to ICU

The kidney plays a vital role in maintaining internal homeostasis through waste elimination, electrolyte and acid-base regulation, and endocrine functions. Recent advances in renal biomarkers have expanded our understanding of kidney physiology by providing detailed insights into the complex mechanisms underlying renal function and injury, beyond traditional measures like serum creatinine and urine output. These novel biomarkers reflect distinct physiological processes, including glomerular filtration, tubular cell stress response, tubular cell damage, inflammation and repair processes within the kidney. Markers such as cystatin C and proenkephalin serve as more reliable indicators of glomerular filtration rate (GFR), almost unaffected by confounding factors like muscle mass, thus offering more reliable information about renal function than serum creatinine. Biomarkers including Tissue inhibitor of metalloproteinases-2 and insulin like growth factor binding protein 7 (TIMP-2 and IGFBP7) reveal early cellular responses to stress by indicating G1 cell cycle arrest in tubular epithelial cells, a process intended to provide protection against injury. Neutrophil gelatinase-associated lipocalin (NGAL) and related molecules provide information on tubular damage and the kidney's acute damage responses. Chemokines like C-C motif chemokine ligand 14 (CCL14) and CXCL9 highlight the role of immune cells in kidney inflammation and tissue repair, reflecting immune-mediated aspects of renal physiology. In addition to molecular and cellular biomarkers, urine microscopy can provide insightful information about tubular cell health. Further, advanced imaging techniques such as multiparametric magnetic resonance imaging (mpMRI) enable non-invasive evaluation of renal perfusion, oxygenation, and fibrosis. mpMRI provides spatial and functional data that deepen our understanding of renal tissue dynamics and the progression from acute injury to chronic kidney disease (CKD). Together, these biomarkers offer a multidimensional view of kidney physiology, distinguishing between functional changes, cellular stress responses, and structural injury. By illuminating the diverse biological pathways in both healthy and injured kidneys, they enhance our knowledge of renal pathophysiology, indicate underlying mechanisms, enable the identification of specific types of AKI, and provide opportunities to stratify patients for intervention trials.