Accuracy of First Response: Early Result pregnancy test?

My girlfriend is 7 days late and is almost always within a day or two with her period. Last night (I realize morning is probably better), she took a First Response: Early Result pregnancy test. I bought this one because of the sensitivity. It was a clear negative. We'll do another test in a few days if her period doesn't come, but, needless to say, I'm still concerned. She's never late, and the test didn't give me the peace of mind I had hoped for. So my question is what are peoples' experiences with this particular test, especially when not using it for early detection but for detection a week after the missed period?