The system is pretty much a crock for any drug test. amandalouise gave some numbers relevant to meth in particular and note that urine and sweat are never mentioned. It's all about how long the drug stays in the blood; when it is cleaned from the blood it is going in the urine and no matter how clean you got the urine, it is getting the continuous addition of the drug until it is out of the blood stream. The drugs in the urine detected in the test have only been there a short while. You might pump water through fast enough to dilute it so they think it was 3 days ago instead of 2 but not fast enough to make it undetectable.