Last time was screened, I had just "emptied" prior to being informed. Went to the clinic and tried with no luck. Could not leave the premises, as that would be a violation. They told me at the desk that I had up to 2.5 hours to submit a test and that I was allowed to consume no more than 40 ounces of water while we waited. Any more than 40 ounces (they told me) dilutes the sample too much. I got the impression that 40 ounces in a given period of time was a mandate/direction of some sort, but didn't get any supporting documents for proof. I drank my 40 and waited about an hour. Sample given and out the door.
On another note, I'm pretty sure you have to be on the clock/compensated for going to give a mandated test if that test is an essential function of your job. You'd run into liability issues if you drove your own vehicle to a test and got into an accident. Conducting company business needs to be compensated and participating in a drug screening is essential to your job. The only exception would be a pre-employment drug screening...you aren't compensated for that for obvious reasons.
On another note, I'm pretty sure you have to be on the clock/compensated for going to give a mandated test if that test is an essential function of your job. You'd run into liability issues if you drove your own vehicle to a test and got into an accident. Conducting company business needs to be compensated and participating in a drug screening is essential to your job. The only exception would be a pre-employment drug screening...you aren't compensated for that for obvious reasons.