I don't really know for sure, but I do know that the drs around here seem to think they are very reliable. When I got pregnant the first time they didn't even second guess it. They gave me "the exam" and the very last thing they did was take blood to make sure I was pregnant, I think because it is required. They didn't even ask me to stick around for the results, but I did just to make sure it turned out positive and it was. They did the same thing with my second baby. I thought it was kind of weird. You think they would have you take a pregnancy test the first thing when they see you, but my dr didn't.