Determinants of Intradialytic Phosphate Removal in Hemodiafiltration From Real‐World Clinical Practice

INTRODUCTION: Hyperphosphatemia is frequent in hemodialysis (HD) patients and associates with cardiovascular mortality. Parameters of online hemodiafiltration (OL-HDF) prescription influencing phosphate removal are poorly understood in real-world clinical settings. METHODS: We conducted a prospective observational study including chronic HD patients dialyzed at a single UK university hospital between December 2021 and July 2023. Phosphate removal was quantified from spent dialysate during a midweek session. RESULTS: Dialysate phosphate was measured during 161 dialysis sessions in 128 unique patients (19 on HD and 109 on OL-HDF), with mean age 63.3 ± 16.2 years and 77 (60.1%) men. Mean intra-dialytic phosphate removal was 25.6 ± 14.3 mmol per session. In multivariate analysis, pre-HD serum phosphate concentration, session time, and urea distribution volume (V) were positively associated with intradialytic phosphate removal. A significant interaction between session time and pre-HD serum phosphate concentration indicated that longer sessions provided greater benefit to patients with higher baseline phosphate levels. Conversely, dialysis modality (OL-HDF vs. HD) and substitution volume were not significantly associated with phosphate removal. CONCLUSIONS: OL-HDF and substitution flow do not demonstrate a measurable beneficial impact on phosphate removal in a real-world setting. Session time, pre-HD serum phosphate concentration, and urea distribution volume (V) are the most relevant determinants of phosphate clearance in real-world clinical practice. Session duration emerges as the most effective modifiable factor for controlling phosphate burden in dialysis patients, particularly for those with higher pre-HD phosphate levels.