From everything I've read, they are not the most reliable choice as far as pregnancy tests go.
Did you ever retest? Hoping and praying you got a BFP!
I am 1, maybe 2 days late, and I took a First Signal test this afternoon, not with my FMU, and I had a faint but definite positive... So I still would like to confirm with my doctor, but according to the late of my last cycle, I'm 4w 5d along. I'm fairly impressed.
I used these tests to monitor my beta levels dropping following a miscarriage, and I still had a faint but positive line when my beta levels were at 13...I think they are more sensative than people give them credit for.