A catheter-flushing extension tube for maintaining the patency and sterility of the lumen of an indwelling catheter. The system is comprised of an extension tube in fluid connection with an indwelling catheter; the extension tube defines an internal volume and at least one sealed proximal terminal for intermittent connection with an external fluid source. The extension tube is configured such that the internal volume of the tube can be progressively reduced at a plurality of different times so that the extension tube itself provides the source of catheter flush solution for intermittently flushing the catheter.