Hi there, there's a good study on this at Fertility Friend at:
http://www.fertilityfriend.com/Faqs/...-pregnant.html
I got a negative test result before i got my positives, but i was less upset about it because i had read this information -- basically you can start testing 4 or 5 days before your period is expected, but lots of people will get false negatives then -- it's much more likely to get a "true" reading right around when your period's expected.
this study does not specify what type of kits were used... also there's a lot of good info to read at
www.peeonastick.com that i found helpful/reassuring.
JD