Backfiltration: does it occur in membrane plasma separation?

Backfiltration in hemodialysis refers to local filtration of dialysate into the blood compartment. This investigation was performed to examine whether there is a similar backfiltration phenomenon in membrane plasma separation. It was shown experimentally in hollow fiber devices that there is a flow of plasma through the filtrate side parallel to the blood flow inside the fiber. This bypass flow includes a backfiltration of the plasma from filtrate to the blood compartment near the end of the module. The following experimental results show the existence of bypass flow and backfiltration. In the case of no net filtration, these findings were made: 1) the existence of an offset of mean transmembrane pressure and 2) the blood side pressure drop in ACD-blood was less in impermeable fibers than in permeable ones. Filtrate pressure is higher than module outlet pressure at a wide range of filtration rates. In summary, backfiltration is not restricted to zero net filtration but occurs under the conditions of clinical use. Therefore, backfiltration is a crucial issue for device safety, because any contamination or wash-out from the membrane can reach the blood side.