I mean the ones that are from four days before your period is due?
I have got a fairly pretty evap on one and my period is due on Sunday or Monday. I went out intending to get an FRER but came back with the cheapy version from Morrisons, as this baby needs to be done on a budget 
So I just wondered, iffff I ended up hypothetically going and getting another pack of these fellas and still don't get a positive tomorrow, or Friday, does that mean definitively out of the game or are they just basically useless?
Thankyou kindly 