Elevated p-Cresyl Sulfate Levels Are Associated with Impaired Endothelial Function in Patients Undergoing Long-Term Peritoneal Dialysis

p-Cresyl sulfate (PCS) is associated with endothelial injury and adverse cardiovascular outcomes. However, its association with endothelial function in patients on peritoneal dialysis (PD) remains unclear. In this cross-sectional study, 82 patients receiving PD were enrolled. Serum PCS concentrations were quantified using high-performance liquid chromatography–mass spectrometry. Endothelial function was evaluated by digital thermal monitoring (DTM), and vascular reactivity was stratified based on vascular reactivity index (VRI) values into good (>2.0), intermediate (1.0–1.9), and poor (<1.0). Overall, 46.3%, 43.9%, and 9.8% of participants had poor, intermediate, and good vascular reactivity, respectively. Poor reactivity was associated with a higher prevalence of diabetes (p = 0.018), old age (p < 0.001), higher waist circumference (p = 0.013), serum C-reactive protein levels (p = 0.010), and PCS levels (p < 0.001) and lower diastolic blood pressure (p = 0.032) and serum creatinine levels (p = 0.005). Higher serum log-transformed PCS levels were associated with reduced VRI after adjustment for covariates (p < 0.001). In multivariable models adjusted for potential confounders, PCS was independently associated with poor vascular reactivity (p = 0.029), with consistent findings observed across penalized regression analyses (all p < 0.001). An inverse relationship was observed between serum PCS levels and endothelial function in patients undergoing PD.