I think it's only 50% accurate 2 days before, I know the day before it's only 70% accurate....
I took 4 AFTER my expected period, and all were negative. Take one after you miss your period and if you end up being late a week or so go to the doctor for a blood test.
I took the test before my period and it came back negative. I didn't get a positive result until the day of my missed period. Sometimes it won't even give you a positive result until a week after you missed period.