AIUI, the instructions say, any visible line in that place on the home test for SARS-CoV-2 is to be treated as a positive test. On the other hand, you're told to check the test only a certain short period after performing it. In that case, what it looks like the next day doesn't count either way.

These could seem trivial points when the lives of your loved ones are threatened by this curse. Nothing I could say will help with that.

Robert Carnegie