Fresenius - Hemodiafiltration

Hemodiafiltration, or HDF, is a renal replacement modality that combines diffusion and convection to improve removal of molecules in the middle molecular weight range versus hemodialysis. Like hemodialysis, blood and dialysate flow in opposite directions in the dialyzer, cleaning the blood of toxins and excess fluid.

While diffusion allows solutes to passively filter through a high-flux dialyzer down a concentration gradient from blood to dialysate, convection uses pressure to actively “drag” larger, potentially uremic molecules, such as Beta-2-microglobulin, across the dialyzer into the ultrafiltrate. During an HDF treatment, approximately 20-30% of the plasma water is removed by ultrafiltration.

The convection volume, or total ultrafiltration volume, consists of the substitution volume plus the net ultrafiltration volume for fluid removal. The substitution volume is the amount of replacement fluid infused into the patient’s blood line to maintain fluid balance. This fluid must be sterile and nonpyrogenic. In high volume HDF, a large volume of plasma water is ultra filtered and must be replaced.

In online HDF, potable water undergoes extensive treatment with carbon filters, reverse osmosis, and sterilizing ultrafilters, with one ultrafilter being installed into the circuit before the site of infusion. This allows delivery of sterile, non-pyrogenic fluid to the patient and use of online substitution fluid, instead of saline, for priming, infusion, or as a bolus administration.

Commonly used HDF treatment modalities include pre-dilution and post-dilution HDF. During pre-dilution, substitution fluid is administered before the dialyzer, meanwhile, in post-dilution, the fluid is administered after the dialyzer.