I’ve been ttc for 2 years, nothing…

I found an old pregnancy test, it expired 2 months ago. I thought I would wee on it just for the fun of it, didn’t think anything of it. It’s come up with a v v faint line. What is the accuracy of this? I know it’s invalid as it’s expired, I just wondered why they expire ??

Someone please help calm my nerves. I will go out and buy a new one BUT I’ve discarded FMU so will have to not drink anything for a few hours which will kill me.