Here is the info that I came across years ago that initially alerted me to the fact that OPKs can be used as pregnancy tests. Just to clear up any misconceptions here that it’s not possible.
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.
Elaboration: a lot of women e-mailed me expressing surprise at this. Keep in mind that HPT’s are often more sensitive than advertised. First Response Early Result was found, in one recent clinical trial, to detect consistently as low as 12.5 mIU/mL of hCG, and sometimes as low as 4.5! OPK’s are never more sensitive than advertised (they are detecting a hormone “surge”; if they are too sensitive they will turn positive from the normal all-the-time presence of LH in the female body.) The most sensitive OPK’s on the market are 20 mIU, and some are sensitive to 30 or 40 mIU. OPK’s may have a second line in the presence of less hormone– the sensitivity refers to the threshold at which it turns positive.
So, if you are comparing a sensitive (20 mIU) OPK to some less-sensitive HPT’s (sensitive to 25-50 mIU) then yes, it’s true an OPK may turn positive before an HPT. However, the most sensitive HPT’s available are more sensitive than the most sensitive OPK’s available.
Most importantly, OPK’s are not purified as well as HPT’s are. Therefore, they are more prone to “errors” and positives do not always mean that either LH or hCG was detected. In other words . . . they’re simply cheaper, shoddier tests.
Finally, research has shown there are actually different kinds of hCG. In some cases (most common in early pregnancy, or in pregnancies with chromosomal abnormalities) the hCG molecule may become “nicked” or “cleaved” (partially or completely separating into its alpha and beta units.) Ovulation tests may not recognize (cross-react) with all types of hCG molecules, and may be falsely negative during pregnancy. (An in-depth explanation of this is coming soon!)
At the end of the day, a positive result on an HPT means you are pregnant. A positive result on an OPK could mean you are near ovulation, pregnant, or the test is picking up an entirely different hormone or element. Or it may be negative even if a woman is pregnant. With a positive HPT there is no doubt; a positive OPK may provide a clue, but doesn’t really give you an answer for sure.
So, my personal rule of thumb: use HPT’s for pregnancy detection, and OPK’s for ovulation.