I found this on another site for you....
WIth OPKs, they detect a sudden increase or "surge" of LH. The typical amount of LH that is in the body at any given time is between 5-20 mIU. Therefore an OPK is looking for a surge above this typical amount. Most OPKs look for a level around 30-40 mIU. However, there are some tests that detect lower levels of LH. You can easily see how testing with a more sensitive OPK could result in a false positive.