Why do faint positives darken after test window?

Question

Anyone know why a faint positive would darken well after the test is taken? I looked back at a faint line and an hour or two later it’s darker than it was initially. Confused.

Answer

My gynecologist called bullshit on all of the evap line nonsense. She informed me that if I take a pregnancy test even if I don't see a positive in the time limit and see one a couple hours later to make an appointment with her. BUT that line HAS to be either pink or blue depending on the test I'm taking. If it's grey then no matter how dark it is, it's negative. A positive hours later may only be a chemical pregnancy, but she still wants to see me because at least it's something. If you have a coloured line hours after you took the test the urine has repeatedly passed over the coloured strip till that HCG threshold was passed to colour the strip. If it's grey then it's just a wet strip.

Answer

All faint lines get darker as they dry, not exactly sure why.

Why do faint positives darken after test window?

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