Meg, I used serum and got a clear BFN (when I wasn't pg). But using serum was a PITA. :/
For ME, there's always a light line that comes up with urine on the combos. But that's because they are so super sensitive, they detect the 2-3 miu of hCG in my urine (that is always there). So, I've pretty much gotten used to the, "I can stare realllly hard in natural light and see something" line and know it's not a true BFP. But it's seriously HARD to see, lol. They detected my first surrogate pg at 7 dpo, and at 10 dpo my beta was 59. My FRER and Answer tests began showing light BFPs at 8 dpo.
TS x 3 - 03/08, 06/10 & 06/12