Substitution and Menstruation

I was considering going the substitution route. What happens if the donor is female and she is menstruating heavily on the day of the test? Has anyone who is reading this been in that position before? I know that testers are not permitted to test for anything but what the drug metabolites themselves ... however, this contingency gives me pause.

On a related note, should this circumstance arise, do any women here know of ways to keep the blood and urine separate even during heavy menstruation?

Sorry for the icky questions :)

The rule of thumb for urine samples while menstruating is that you wipe first, then insert a fresh tampon, and then begin urinating. If you don't use tampons, just wipe well first. Since the blood isn't a constant flow the entire duration of your bleeding cycle, wiping will prevent excess blood from entering the cup.