what do opks actually measure?

[callmedan]

I know how they work (darker second line during ov) but what do they actually measure? All I can find is LH but what is that? Is it something that is produced all the time? Would I get any kind of line on an opk whilst on the pill?
Thanks

 

[katherinegrey]

LH is in your system all the time, opk's measure the surge that occurs before ovulation, I always get a line, even on the pill.

 

[callmedan]

Thank you