Sigh. I have no problem interpreting a postive vs negative OPK. I'm a systems
engineer, so reading instructions is second nature to me. I am also not asking you what the results are. I know that they are positive for LH surges.
Maybe I didn't write what happened clearly:
OPK CD 16 : Test line SAME DARKNESS as the control line (positive)
OPK CD 17: Test line SAME DARKNESS as the control line (positive)
OPK CD 18: Test line significantly DARKER than the control line (positive)
OPK CD 18 (later in day) Test line slightly LIGHTER than the control line (negative)
OPK CD 19: Test line SAME DARKNESS than the control line
I decided not to throw out the first CD 18 sample, in order to retest with Clearblue digital. Came back "positive" for the first CD 18 sample.
Also did a concurrent test the CD 19 sample -- the Clearblue came back "negative", even though the CVS brand came out positive..
After some research, I believe that I found the answer.
CVS brand used to be white-labeled Answer test strips, but this is no longer the case. The newer formulation is more sensitive -- it takes 20mIU to trigger a positive. (my sister works for the FDA and looked it up for me today)
Clearblue Digital is a little less so -- either 30 or 40 mIU to trigger a positive.
This means that someone whose cycle decided to have either a slow ramp up that month, OR failed to ovulate and got a quick backup surge the next day, could show positives for 3 or more days.
It also appears that many women on this board have never heard of an overlapping LH surge, or secondary surges. The 'go by first positive' method is not a hard and fast rule.
Because I'm on vacation and have no life, I will take pictures and post this info in another thread.
(Edited for readability)