How do OPKs actually work?

[Bumpsadaisie]

Am stumped as to how OPKs detect a "surge" in luteinising hormone?

On the day that you get your surge, how do they know that the LH levels were lower on the previous day?

Or is it just that there is no LH until you ovulate and therefore each test just looks for any signs of it?

What I am really getting at is, do I need to test in the "run up" to ovulation in order for it to spot ovulation, or if I happened to test for the first time on the day of the LH surge, it would detect it anyway?

Many thanks!
Bumps x

It detects LH levels above a certain threshold. Not sure the exact numbers, but say the detection level is 15iu, unless your baseline is that high, it will pick up when you LH goes above that level.
HOWEVER, it goes up in a spike,so falls as well, and you can pick up the surge on the rise and fall, so you ought to test for a couple of days before so you get some negatives, then you know the first time it is positive is your surge. Otherwise it may be picking up a high baseline or the end of your surge.
Blimey, that makes no sense, sorry. Basically OPKs work really well for people with low baseline LH/FSH and regular cycles (who probably don't really need OPKs).

thanks freelance I've wondered the same. Does that mean that the lines get lighter as your LH gets lower?