This is one of the many urban myths that are still floating around out there about drug testing. Maybe it was true 20 years, when drug testing was first introduced, but labs are on the nitrates (common additive to "detox" drinks that do not work), niacin (does not work but will make you sick), golden seal, green tea, etc.

Labs can now check for bleach, as well as many of the ingredients in the detox drinks by doing integrity checks of the sample for creatine and specific gravity.

Provided the collector follows regulations, he/she will make you wash your hands prior to going into the john to wash off any potential adulterant, such as bleach.