Hi,

I don't think leaving the test in sample for too long would give you a false +ve (I think more likely a false -ve as too much urine could wash away the dye).

Personally I'm not a huge fan of Pregnow. I got a BFN using Pregnow at 15DPO and a STRONG BFP using First Response at 17DPO (Pregnow are supposed to be 10mlu and First Response 25mlu). I've also heard of others getting false +ves with Pregnow. I think Pregnow is probably not the most reliable......