I have irregular cycles, so I've never really known when to test. They vary from 30 to 36 days. I tested with a First Response at like day 30 and got a negative, then I tested again a week later because I still hadn't started and got a positive. So it is possible to get a negative and then a positive!!
I used a first response early detection 4 days before missed period and got negative.....did it again 2 days later (not even first morning urine) and got my positive!!! :). Good luck!! I read that first response early detection is the most sensitive test.
OPK's are..weird. I've never used them myself, but to my understanding you need to use them 2x a day, start early in your cycle and that when it shows a positive it means you'll ovulate within the next 12-24 hours. I know I've seen women who get it early in the morning, and then at night it's gone. It's possible you could have, but it's so difficult to say.
My periods are also irregular. My period from May to June was 35 days. My period in June started in June 3rd and lasted until June 8th. Starting on June 19th (CD 17) I started tracking for ovulation using OPKs. I tested through June 24th (CD 22) and then I didn't track June 25th or 26th due to camping. On June 27th I had EWCM and my boobs were extremely swollen. However when I took a OPK it came up negative. Could I have surged on the 25th or 26th, but ovulated on June 27th? I wish I knew....
On July 4th, 5th and 6th I felt as though I was getting my period. I was extremely irriatable and having anxiety plus feeling bloated. However since the 6th I have felt normal for the most part. I am just confused and wish I knew what my body was doing!
They can be used, but the odds that it will be accurate are something like 50%. If you check out the box, it gives a guideline of how accurate they truly are. The sooner you test, the less accurate it is. The longer you wait, the more accurate. It also makes a difference if you test with first morning urine or not. If you do, you're more likely to get accurate results sooner. If not, your chances are high it will show negative. In early pregnancy or early testing, you should always use first morning urine.
As for when I tested, I found out when I was 6 weeks because I have irregular cycles and missing a period wasn't a big deal for me.