its best to always test at the same time every day in your fertility window. i tested at 5 am and got a strong positive on opk because i figured my urine might have a better chance of showing my lh if its concentrated. but then again, i tested throughout that same day and still got the same line darkness so maybe its not exactly the concentration of lh that matters. either way, my first month trying to use opks were awful. they always looked positive so i didnt know which test on which day to believe. and the second month i didnt even get a positive but i did skip a day in testing so that day i skipped was probably the one day that i was going to O. the third month as i got used to looking at them, i got a strong positive like no other ive seen. very distinguishable. i felt like a pro at opks! i dont like the clearblue digital ones i used wondfo ones.