Just started doing CRRT at my hospital and I had a question that has been nagging at me. We mostly do CVVH on out patients and I know when we do that we can either add in the replacement fluid pre filter or post filter. What I do not understand is that if we add replacement fluid post filter then how can the solutes be removed by convection? Correct me if I'm wrong (which I probably am), but I was under the impression that as the replacement fluid is added it drags the solutes out of the filter by convection. Now this makes sense if it is added pre filter, but if the replacement fluid is added post filter then it cannot drag anything out of the filter at all since it is added only after the blood has already passed through the filter. Any help would be greatly appreciated. Thanks.