Reliable?

Does anyone know exactly how reliable the at home test strips are? I bought some at a local headshop to test my dilution and see if I can come up clean. Well.....I have been doing things such as water, creatine, cranberry supplements, vitamins and such. But this morning before taking anything, I passed the strip test. Unsure if it was a false neg, I did it again this afternoon, AND PASSED! Granted, I did not smoke that much weed this weekend, but could it be???? I bought quite a few of them, so I'm going to keep checking. They have the 50 cutoff for THC just like the quest test I have to take Fri. but if anyone has any opinions on these things, PLEASE share. I don't want to go in with confidence and fail.