SPDL1 and cancer: To compare serum sPD-L1 levels among healthy individuals and patients with various CKD stages, including those undergoing dialysis, a secondary analysis was performed using clinical data and residual serum samples from four distinct cohorts, each prospectively collected for different research purposes: The Vaccine Cohort (2021–2022), the Cancer Cohort (2010–2018), the Dialysis Initiation Cohort (2023–2024), and the Dialysis Maintenance Cohort (2011–2015) included patients on stable maintenance dialysis.