kobe: part of it is that it takes a while to do. The other part is to create doubts in the heads of people who did drugs and think they may have been caught though they may have passed the test. The hope is that they would confess to it.
Now you sorta know.
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"Why do you have to be such a smart ass?" -my mom
but why does it take a while to do? don't you just spin the stuff in the centrifuge, let the residue fall to the bottom and you have yay or a nay? j/k
i'm amazed that technology hasn't sped up this process yet.