I thought that most of the waste products are removed through diffusion rather than ultrafiltration, and that ultrafiltration regulated the amount of fluid removed. Some solute will be removed with the fluid, but it can still diffuse across the dialyzer membrane when the UF is off. The only time there is no waste removal is during SCUF, when the dialysate flow is turned off. I've never heard of having to keep the UF on during dialysis - in fact, we have one patient on whom we do dialysis with the UF completely off throughout the treatment - she's not my primary patient, but I'm pretty sure her numbers are good.
