I thought it was 10 or 25 (big difference I know but can't quite remember) - I got negative at 10DPO and very feint positive at 12DPO and 14 DPO so wanted to keep testing - I ended up going to the "digital" ones that give you a definite answer (but they are only anygood post 14DPO so no help for earlier indicator. I was pregnant though so from that perspective they were right just the lack of surity due to feintness was hard to take.