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Hemodiafiltration : the best of both worlds

HDF combines the elements of two processes, conventional hemodialysis (HD) and hemofiltration (HF). Renal replacement therapy for ESRD is based on two processes: diffusion and convection.

Conventional HD is diffusive; blood is circulated in an artificial kidney machine on one side of a semi-permeable membrane, while a special dialysis fluid is circulated on the other side. Small molecules of metabolic waste seep out into a dialysis solution flowing in the opposite direction on the other side of the membrane, mimicking the kidneys and washing wastes and toxins out of the bloodstream. One major toxin is urea. HD is the most widely used renal replacement function technology for ESRD.

Hemofiltration (HF) or ultrafiltration is exclusively convective, forcing blood through a filter under high pressure. The principle consists of applying a hydrostatic pressure gradient (high pressure on one side, low pressure on the other) across the membrane or filter. This results in an ultrafiltrate (water and electrolytes) on the other side. The quantity of ultrafiltrate lost in this process must be compensated by a matching infusion of replacement fluid. HF is used primarily in continuous mode and in acute care or intensive care.

In HDF, the diffusive component of HD is combined with the convective component of HF. As is the case in HF, the excessive loss of liquid must be compensated by the reinfusion of a sterile and apyrogenic (not producing fever) fluid. Recent developments have led to the "on-line" production of large volumes of ultrapure liquid of high quality. This has led to higher quality physicochemical and microbiological properties in these solutions, in comparison with HD.

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