I have a 40+ day cycle this time, been using opk since cd 11. Was getting frustrated with the generic Amazon strips and bought answer brand strips from Walmart on cd 26. I tested at 2 pm with just the answer strip. Got a negative. Tested the 2 brands at same time same urine sample at 10 pm and got 2 diff results. Answer brand was positive and cheap Amazon brand was negative. Tested with answer brand again this morning. Still positive. I'm so glad I bought the new brand. Otherwise I would have missed my surge.