If you suspect you are having a miscarriage, a pregnancy test is not a reliable way to find out for sure. It's possible, especially early on to have a negative, or even lightly positive pregnancy test if you have miscarried. But more likely, it will take quite awhile for the hcg to diminish from your bloodstream, and thus give a negative pregnancy test. After my miscarriage, I had positive tests for around 3 weeks after my D&C.
The best thing to do if you suspect you are having a miscarriage is to call your dr's office and demand an ultrasound. Your insurance should pay for it if it is for diagnostic purposes, which it is. On one hand, mother's have intuition that cannot be denied... on the other hand, we all tend to panic when we know there is a baby involved, and sometimes we worry about nothing.
Listen to your heart, you'll figure out what to do, but don't expect the pregnancy test to diagnose a miscarriage.
Stay blogged and get Preggot!