This is what I read and it might explain it a bit:
OPK's as HPT's:
using an ovulation test to confirm pregnancy
Overall-- yes, it can be done. Ovulation predictor tests (known as OPK's because they are normally sold in a set of tests known as a "kit," thus the K) will show a positive result when a woman is pregnant, as well as when she's ovulating.
Specifically-- while it works most of the time, there are good reasons to avoid using an OPK as an HPT for diagnostic purposes. If you want to pee on any stick that will stand still after you know you're pregnant, just for fun, have at it. But I would not recommend using an OPK in place of an HPT overall.
Reasoning-- OPK's detect LH (luteinizing hormone) which is the hormone associated with ovulation. Pregnancy tests detect hCG, the hormone associated with pregnancy. LH and hCG are, at a molecular level, nearly identical. hCG has a beta subunit, meaning it has an extra little "doodad." To use a stupid but easy to understand example, LH and hCG are identical twins, except that hCG wears a funny hat.
An OPK tests only for the part of the molecule that LH and hCG have in common (the "face" or "body" of the identical twins.) Essentially an OPK is saying:
So an OPK will turn positive when it detects either of the "identical twins"-- ovulation or pregnancy hormone.
The reverse is not true, however, because an HPT tests for the part of the molecule that is unique to hCG (the "hat.") So an HPT would say:
Therefore, a pregnancy test will turn positive only in the presence of hCG, whereas an OPK will turn positive in the presence of hCG or LH.
Now, it's important to note that OPK's work differently than HPT's. A pregnancy test will develop 2 lines only if hCG (pregnancy hormone) is detected. Thus, "a line is a line" when determining a positive HPT. OPK's work differently. An OPK has a "control" line and a "test" line, just like an HPT. Unlike an HPT, however, the mere presence of a "test" line does not mean the test is positive. The test line must be as dark as, or darker than, the control line to be a positive result (meaning that a surge was detected, rather than the ordinary amount of LH usually found in your urine every day.)
This means that there is already some ambiguity involved in reading an OPK's results. Sometimes the line is almost as dark as the control line, but perhaps not quite as dark. Sometimes only the edge turns dark, or the top half of the line is darker than the bottom.
Additionally, OPK's are not as sensitive as a lot of HPT's are. This means that, if pregnant, you are likely to get a positive HPT earlier than you would get a positive OPK.
