The new ins book states to flush with 10cc of nss post blood draw, we have always flushed with 20cc post blood draws but I do not remember where we got that from.
You can always use more unless you have someone on a severe fluid restriction. When you look at the primimg volumes of the lines in adults you can easily see that 10 ml is sufficient. We wanted a different amount than our normal flush volume to help alert the nurse that this step should not be deleted. Sounds strange but many of our PICCS were NOT getting flushed at all post blood draw and often they would draw the blood...with flushes not in hand and then when they had time they would come back and flush or try to flush the PICC or CVC. Hard to believe but TRUE.