I ve always only used first response. A faint line is still positive, I can see the line. I had a faint line on one test and ended up pregnant with my first&second. I dont remember how many days past I was though, probably around 12. I just got a BFP on one I took this weekend with a more pronounced line but I am also 25dpo. I can't speak for digital. You can always wait a bit longer and re test to see if it gets darker.