First Response reckon that they can detect preg hormone up to 4 days before your A/F but there is a stat on the back something like "at 4 days before period detected 65%, 3 days 75%, 2 days 85%, 1 day 90%" so its not that accurate if you get a BFN you could still be pregnant. I used one 4 days before, but used early morning "sample" which gave me a BFP.... HTH.