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.
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