Not very accurate. Even the "early result" tests can only test up to 5 days sooner than your missed period, and they say on the box that the results aren't nearly as accurate as waiting until you miss your period. You also could have ovulated late. In that case, taking an early test woudl definitely show up negative.
If I were you, I'd wait until maybe a day or two before you msis your period and take a test.