If I am not mistaken, fentanyl isn't fully metabolized into nortfent, it's excreted in urine unchanged, and the test strips would be testing for it's presence. I have used urine cups to test pills for meth and I have used dipsticks to test for fent.Putting a test strip in urine will only register metabolites......if that.
Urine doesn't show the actual chemicals in something it only gives the waste and byproducts of that particular chemical. So if there is fentanyl ( or anything else in something) testing the actual product in front of you is the only way you can tell. It seems logical that the urine would show you what's in the sample but it won't.
I'm not a chemist or biologist so I may be wrong but I don't want people to think that they can test their drugs by putting the strips in urine as opposed to actually testing the actual product.
It doesn't work for all drugs, but for some drugs you can just follow what the tests are designed to detect, and also look at what a drugs metabolism will be.
Apparently based on the post above, the manufacturer also states this.
Also, you are right not to just trust some random person on the Internet who says yes. In this case, however, I knew this to be true. I shouldn't have been lazy and not offered more explanation than I did.