A catheter for use in the extracorporeal treatment of bodily fluids comprises a catheter body having a withdrawal port, an infusion port, and a plurality of lumens therein. One of the lumens comprises a withdrawal lumen for transport of fluids withdrawn from a body vessel through the withdrawal port to an extracorporeal treatment unit, such as a dialyzer. Another lumen comprises an infusion lumen for infusion of treated fluids from the extracorporeal treatment unit through the infusion port into the vessel. The withdrawal port is positioned proximally from the infusion port along a length of the catheter body, and includes a generally helical profile.