At what duration is phosphate removed during hemodialysis (HD)?

Phosphate Removal During Hemodialysis: Duration and Timing

Phosphate removal continues throughout the entire hemodialysis session, with only approximately 25% removed in the first hour despite serum levels normalizing early—the remaining 75% is removed during the subsequent hours, making extended treatment duration critical for adequate phosphate control. 1

Phosphate Removal Kinetics

Time-Course of Removal

• The widespread belief that phosphate is primarily removed in the first hour of hemodialysis is a misconception—while serum phosphate concentration normalizes within the first hour (47.4% reduction), actual phosphate mass removal during this period represents only 25% of total session removal 1

• The remaining 75% of phosphate is removed during hours 2-4 of the dialysis session, despite lower serum concentrations, due to mobilization from intracellular and tissue compartments 1

• Serum phosphate reduction rate is similar at 1 hour (47.4 ± 14.3%) and 4 hours (45.1 ± 10.8%), but total mass removal continues to increase throughout the session 1

Impact of Treatment Duration

Extended Session Length

• Increasing dialysis duration has a much greater impact on phosphate removal than increasing frequency alone 2

• Weekly phosphate removal increases significantly with longer session duration: 3,007 mg with 4-hour sessions versus 3,400 mg with 5-hour sessions (p < 0.02), even when Kt/V is held constant 3

• Total weekly dialysis hours exceeding 24 hours per week, distributed over at least 3 treatments, appears necessary to control phosphorus levels in most dialysis patients 2

Specific Duration Thresholds

• In the Tassin experience using 8-hour sessions three times weekly (24 hours/week total), approximately one-third of patients no longer required phosphate binders 2

• Nocturnal dialysis using an "every-other-night" strategy (28 hours/week) achieves similar phosphate control to the Tassin experience 2

• Nocturnal dialysis given 5-6 times per week removes the need for phosphorus binders in almost all patients and often requires phosphate addition to dialysate to prevent hypophosphatemia 2

Frequency Versus Duration

Short-Daily Dialysis Limitations

• Short-daily dialysis schedules (1.5-2 hour treatments) show disappointing phosphate control despite increased frequency, as patients often increase dietary phosphate intake that compensates for the modest additional removal 2

• There appears to be a minimum hemodialysis duration below which increases in frequency cannot adequately compensate to achieve normal phosphate levels 4

• Short daily hemodialysis (2-3.75 hours, 5-6 sessions/week) decreased serum phosphate from 1.99 mM to 1.27 mM, but did not normalize levels in all patients 4

Optimal Strategy

• Increasing the weekly number of dialysis sessions is more effective than prolonging individual session duration for phosphate removal, as dialysis effectiveness decreases over the course of each session due to declining plasma phosphate concentration 2

• However, this recommendation applies primarily when comparing very frequent (daily) short sessions to less frequent longer sessions—the evidence clearly shows that within a thrice-weekly framework, longer duration is superior 2

Factors Influencing Phosphate Removal

Session-Related Factors

• Pre-dialysis serum phosphate level is the strongest predictor of phosphate removal (r=0.455, p=0.001)—patients with pre-dialysis phosphate ≥5.5 mg/dL remove 975 mg versus 776 mg in those with <5.5 mg/dL 1

• The volume of blood passing through the dialyzer during the entire session correlates with phosphate removal (r=0.343, p=0.019) 1, 5

• Vascular access type matters: arteriovenous fistula achieves 720 mg/treatment versus 620 mg with tunneled catheter (p=0.023) 5

Factors With Limited Impact

• Increasing Kt/V within a thrice-weekly framework while holding treatment time constant (by increasing blood flow or dialyzer clearance) has only mild to negligible effect on serum phosphorus levels, as phosphate decreases to low levels early in dialysis 2

• High-flux versus low-flux membranes show no significant difference in phosphate removal (700 mg versus 640 mg, p=0.280) 5

• Dialysate flow rate (500 vs 700 mL/min) does not significantly influence phosphate removal 5

• Online hemodiafiltration does not increase phosphate removal compared to standard hemodialysis (733 mg, p=0.383) 5

Clinical Implications

Minimum Treatment Duration

• The minimum dialysis treatment time for 3 treatments per week should be 3 hours to achieve adequate salt, water, and phosphate balance 2

• This 3-hour minimum reflects both the need to maintain stdKt/V close to 2.0 per week and the difficulty achieving adequate phosphate and volume control with shorter treatment times 2

Monitoring Considerations

• Pre-dialysis plasma phosphate levels should be maintained around 50-70 μmol/L in dialysis patients, though this may vary between laboratories 2

• Specific attention must be given to phosphate control in patients receiving intensive dialysis, as hypophosphatemia may worsen bone disease and mineralization defects 2

• For intensive dialysis regimens, monitor serum phosphorus, calcium, potassium, and magnesium levels frequently, targeting phosphorus in the lower end of normal range (2.5-3.0 mg/dL) 6