hope, lulu is right. I didn't test all at once. I think I had a like 4-5 days in between each test. So if your lines aren't changing too much don't worry. But if you test like a week later, or 4 days later and the lines are lighter, you probably want to test again, and if it's still lighter, test again or go to the doc. Like everyone said above, it's not science, but it's helpful.