I have a similar question to mutedfusion's question.

I took a lab test on December 10 after failing a home test the previous day. The lab results showed I was passing. Just to be sure I took a home test kit on December 18 (2nd piss of the day), and passed it, but the line was a bit more faint than I had hoped. I took my drug test for real on December 23. Because they were closed for Christmas Eve, I won't know till Monday or Tuesday.

Would it be possible to fail the test after first testing clean 13 days prior to the real test? After being clean for 30 days.