I can't speak to HD catheters but I suspect they are similar to PICC catheters used for lyme and oncology. The problem is there is invariably going to be some lock solution that enters the body for almost all patients. The volume of the catheter is known for PICC lines. The catheter's inner diameter is known and the length is marked on the outside of the catheter. So, the nurse knows (if they are paying attention) what volume of lock solution is needed to fill the catheter. However, there will likely be small amounts of lock solution that enter the body every time the line is used. There are 2 reasons for this. First it's difficult even using a syringe to push in the exact right amount of lock solution. You don't want to put in too little because clots can and will develop at the end of the line inside the body. The second reason is there will sometimes be a clot that forms at the end of the line and the way to get rid of it is often to push the lock solution into the body to push the clot away.
So, does this make a lock solution a drug even by the new standard?