According to one site:

Chlorinated bleach will test negative, and it's the best household additive. In an emergency, Jeff Nightbyrd recommends adding unscented bleach crystals to a diluted sample. It's recommended to grind the powdered bleach to a finer grain. 1/4 teaspoon is recommended for a 60cc sample. For liquid bleach, add six to ten drops. Bleach will throw the pH outside the normal body range; so it may be apparent that the sample was tampered with. Some bleaches foam or leave residue, so experiment with different brands before selecting which to use.

What that means: Get a hold of some of the test strips and find out with your own trials wether it will work or not, and the best way to do it.
Chillum1887 Reviewed by Chillum1887 on . tampering with urine cup I think on the forums i heard once if u brush bleach on your urine cup then the urine in it it will come up negative. Is there such thing i can put in my urine test cup that will make the test come up negative. Its one of those strip urine test ones. Rating: 5