The association between renal medullary and cortical fibrosis, stiffness, and concentrating capacity: an observational, single-center cross-sectional study.

Background: Fibrosis is a common final pathway leading to end-stage renal failure. As the renal medulla and cortex contain different nephron segments, we analyzed the factors associated with the progression of renal medullary and cortical fibrosis.

Methods: A total of 120 patients who underwent renal biopsy at Kawashima Hospital between May 2019 and October 2022 were enrolled in this retrospective study. Renal medullary and cortical fibrosis and stiffness were evaluated using Masson's trichrome staining and shear wave elastography, respectively. Maximum urine osmolality in the Fishberg concentration test was also examined.

Results: Medullary fibrosis was positively correlated with cortical fibrosis (p < 0.0001) and log-converted urinary β2-microglobulin (MG) (log urinary β2-MG) (p = 0.022) and negatively correlated with estimated glomerular filtration rate (eGFR) (p = 0.0002). Cortical fibrosis also correlated with log urinary β2-MG, eGFR, and maximum urine osmolality. Multivariate analysis revealed that cortical fibrosis levels (odds ratio [OR]: 1.063) and medullary stiffness (OR: 1.089) were significantly associated with medullar fibrosis (≧45%). The severe fibrosis group with both medullary fibrosis (≧45%) and cortical fibrosis (≧25%) had lower eGFR and maximum urine osmolality values and higher urinary β2-MG levels than the other groups.

Conclusions: Patients with disorders involving both renal medullary and cortical fibrosis had decreased maximum urine osmolality but had no abnormalities in the urinary concentrating capacities with either condition. Renal medullary and cortical fibrosis were positively correlated with urinary β2-MG, but not with urinary N-acetyl-beta-D-glucosaminidase.