The rule of thumb with ADV testing is that if there is cause to doubt a
result or if there is a positive result then have a different type of
test by a different place done.  Don't panic, don't go to any form of
extreme, and don't readily accept a result when there is room for doubt.
That's it in a nutshell.  There is a lot of info about why this approach
makes sense, who does testing, why tests aren't 100% accurate, etc. in
the Archives of BOTH the FML
 
http://listserv.cuny.edu/archives/ferret-search.html
  and the FHL
http://fhl.sonic-weasel.org
and in ADV websites
http://www.ferretcongress.org for links to those and a lot more
useful stuff in the critical reference section.
 
When ADV testing is under discussion it best to recall that absolutes
fit for almost nothing except the statements that testing makes sense
and that ADV when it takes it off is a real horror.
[Posted in FML issue 4215]