Rationale: The IV catheter has entered the lumen of the vein successfully when blood backflash shows in the catheter. The vein should have been distended by the tourniquet before the vein was cannulated. Client discomfort varies with the client, the site, and the nurse's insertion technique and is not a reliable measure of catheter placement. The nurse should not advance the catheter until placement in the vein is verified by blood return.