MG - I used one of these after getting continually feint lines only on Fortel ultra sensitive even at 16 and 17 DPO (had a couple of spares that I used to see whether the line ever got any darker) - I wanted a definite answer one way or the other by 14 DPO - for me it was a BFP - don't know that they'd be much good before 14DPO though (that's when I used it - cause I'm sure it said to use it only once late)