A catheter assembly for use in extracorporeal treatment of a body fluid comprises a catheter body having a plurality of lumens extending therein. The catheter body has an aspiration port in communication with a first lumen for transporting fluid withdrawn from a body vessel to a treatment instrument, such as a dialyzer, and an infusion port in communication with a second lumen for return of treated fluid to the vessel. The catheter body includes a wing-like flap portion extending radially from the catheter body and defining the aspiration port. The flap portion is configured and arranged to space the aspiration port from a wall of the body vessel.