Your test couldnt have been a DOT test if they did a strip test in front of you.
Under DOT guidelines, no instant strip tests can be performed at the collection site. All DOT sample collections must be split in two vials, initialed, bagged, initialed again, and sent off to a lab for analysis. The only preliminary checks the collector can make is the temp check and looking for unusual sample color or bleach odor.
The DOT guidelines must be adhered to by employers, the collectors, & lab personnel or else the test results are deemed void.

To learn the regulations of collection, processing, analysis, and reporting the results of DOT testing, click here: http://www.dot.gov/ost/dapc/ and http://www.dot.gov/ost/dapc/NEW_DOCS/part40.html?proc

Protect your rights - Know the regs.