Am J Kidney Dis. 2009 Dec;54(6):1108-15. Epub 2009 Jul 19

Study hypothesis:

To determine whether more intensive hemodialysis would improve phosphate clearance and decrease serum phosphate concentrations in large hemodialysis patients.

Study design and study population:

Data extracted from a previously published randomized crossover trial (performed by the same group) designed to assess the effect of increased dialysis delivery on quality of life. Included patients had a dry weight > 80kg, Kt/V < 1.2 on 2 occasions in the previous 6 months or required > 12 hrs hemodialysis per week to maintain adequacy (not volume removal).

Intervention or observation:

Patients received each of 4 dialysis schedules in a randomized cross-over schedule. All sessions lasted for 4.5 hrs.

  1. Standard dialysis 4 hrs three times / week with QD 500 mL/min (30 mins no dialysis)
  2. Increased dialysate flow (QD 800 mL/min) for 4hrs three times / week (30 mins no dialysis)
  3. Increased dialysis time 4.5 hrs three times / week (QD 500 mL/min)
  4. Two dialysers in parallel with QD 800 mL/min for 4 hrs three times / week


Primary end point, secondary end point:

The pre-dialysis phosphate level was the primary outcome.

Secondary outcomes were post-dialysis phosphate level, phosphate clearance estimated from a total dialysate collection and phosphate removal. The sample size was determined for the original quality of life study rather than this prespecified analysis


The use of 2 dialysers in parallel significantly decreased pre-dialysis and post dialysis phosphate levels when compared with both standard and increased dialysate flow methods. Phosphate clearance was also higher using this method, but total phosphate removal was not statistically different comparing any of the methods.

Methodological assessment:

The randomized cross-over nature of the study is strong. The explanation for the apparently paradoxical finding of enhanced phosphate clearance with no change in total phosphate removal is speculative and hard to follow. The authors rightly suggest that equilibration between the intracellular and extracellular compartments for phosphate is the rate limiting step for phosphate removal. However they also comment that the steady state phosphate levels towards the end of dialysis are consistent with mobilization of phosphate from an extracellular pool. Assuming no decline in phosphate dialysability with time, this mobilization would be expected to increase net phosphate removal which the authors failed to show.

Impact on practice:

While it is reasonable to attempt to enhance phosphate removal by dialysis as part of a phosphate control program, the cost of using two dialysers in parallel is prohibitive (even when compared with the new generation of expensive phosphate binders).

- Reviewed by Dr. Ross Morton

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