Are those THC Dip card tests accurate?

I have quite smoking exactly 3 weeks ago so I could pass a upcoming drug test at work. I purchased one of those single thc dip card tests and it says I'm positive. I know the 30 day rule but hell I'm 5'11'' 160.lbs physicly fit and I have a very high metabolism. I honestly can't see how I can still be positive after 3 weeks. Does anyone know if these test cards are even accurate? Iv'e heard about false positives. How can I argue a false positive case If I end up failing the real piss test for work?