Hi Careys,

OPKs test for the LH in your system, you get a surge of LH which triggers ovulation. When you get the surge you will get a positive OPK. Apparently LH is in your system all the time but only surges before ovulation so that would explain why you get the positive OPK on CD5.

I tried it by doing an OPK on CD25 and i got a positive too and i know for a fact that i had ovulated already.

Hope this helps.