OPKs and Expiration Date

[NavyWife84]

Does anyone know the science behind the 30 day expiration date on OPKs? I understand that they aren't supposed to be exposed to the air because of the chemicals or whatever, but do they really expire after a month if you keep the container closed properly? I buy the 20 count packs rather than the 7 count packs because I am afraid of testing too early or too late if I only get the 7 (my cycles usually last between 30-38 days) and O is not very predictable! However, I always have some left over and hate to waste since they are so expensive!!!

I don't want to use expired OPKs and risk getting a false reading, but I really hate to waste! What are your thoughts?