Urine drug tests?

Ok so I was thinking about drug tests the other day. Now, it is my understanding that when a drug test is done urine is collected then a reagent is added to the sample and a Immunoassay is done to determine concentration or sometimes in gas chromatography/mass spectrometry for the same reason or as conformation. Now my question is, since both of these work on concentration in the actual sample, would water dilute this out more?

I know of the practice of drinking a lot of water pre testing for the purpose of flushing your system, but is it actually just diluting the sample to the point where it reads much lower than it is?

And if this is the case could one just add water to said sample to dilute it further, or does water bind with the urea or something making this obvious it was done? Is there a certain ratio of water to urea that would make this obvious as well?

Totally unrelated but I just noticed. I got to love how it now asks if the question has been asked before. Good work guys :)