All pregnancy tests are fallible. If you are in a situation where you can wait, do so for a few days, and then test again. If you are pregnant, the hCG hormone (the hormone the tests detect) will usually double approximately every 2 days. You may still get a negative result up to 4 days past your missed period! Obviously, if you do get your period (keep in mind spotting is not a period), you are not pregnant.
If it is very important that you know ASAP (for instance you are on medications that could seriously harm the baby), have your doctor order a blood test. This can detect a much smaller amount of hCG.
Really, it all comes down to waiting. I know it's incredibly difficult, but it's all you can do with our current technology. Best of luck!
at 11:10 AM on Jun. 24, 2011