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 a__sociated with ovulation. Pregnancy tests detect hCG, the hormone a__sociated 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" 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. So there is already an element of confusion involved.
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.
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.
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. With a positive HPT there is no doubt; a positive OPK can make you hopeful but doesn't really give you an answer for sure.
So, my rule of thumb: use HPT's for pregnancy detection, and OPK's for ovulation.
If you already have your positive OPK and just want to "play" with tests, of course, that's something else entirely ;) So, for fun, ogle my OPK's. (They are Inverness brand ca__sette tests.) The one on the left is negative. It was taken at 9 DPO. I was pregnant, but my baby had not yet implanted (it was therefore too early for a positive test of any kind.) The one on the right was taken at 13 DPO, the day after I got a positive pregnancy test. The test line is clearly darker than the reference line, and is therefore positive.