Well, if blood keeps getting into the test sample, then they might make you use a tampon or give you another date to take the piss test.

Excuse the frank story people, but a co-worker of mines said they had to reschedule her drug test because she got so much blood in the sample, plus she was a heavy bleeder and they didn't have any tampons for her.

I suppose it depends on the lab, but a significant amount of blood can interfere with the test results.