Tampered with by whom? If they are saying you tampered with the sample or the testing, that's not helpful.
Yes, a retest is done of the same sample, usually by a different place. But not always.
They do not normally do a complete retest because the presence of any drug dissipates over time - usually within up 72 hours, unless it is something like marijuana. So a negative test today doesn't rebut a prior positive test.
In my criminal cases I will suggest a hair test. However, the hair test is not considered dispositive. That is because a hair test is able to identify a one time use, but a negative doesn't indicate no use. A negative hair test's value is that it shows the absence of long term or multiple uses over time if it is negative.
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