Now what do you mean by "you might test clean one day and not the next". Just as a background I test myself 14-15 days before my actual test may be given. Like I said before I also have never failed an at home test doing the same thing I do every month: smoke 2-4 bowls every 28-30 days, exercise, eat fiber, etc... Do you not trust the at home test? I usually take the one from Walgreens or CVS.