Why shouldn't you look at a HPT after 10 mins?

[Incacola]

I did a HPT this morning and decided that nothing was developing in the positive line before the 2 mins was up so put it in the bin. I later went back to it and there was a faint line. The instructions very clearly say don't read it after 10 mins. Is that because the test can give a false positive?

I did another HPT later today and it was BFN.

Yes it's because the liquid dries up and leaves a watermark (evaporation line) which can be misinterpreted as a positive line.

Thanks NickNacks, that makes sense.