Is the Test valid if....

I have been using the Clear blue Advanced Digital Ovulation Test Kit, I started testing on CD 8 and was getting the low fertility sign till CD 16. On CD 16 I got a Peak Fertility Sign using FMU (Like always) but after peeing in my cup I forgot to test cause I was running late and left it sitting for 8 hours as soon as I got home I tested and got the Peak Fertility sign. was the test right or did I leave my pee sitting for to long so it might be invalid? also If I test and at first get Low Fertility then I get is High Fertility but never get Peak Fertility will/can it go back to Low Fertility or will it stay at High Fertility till I start my new Cycle?