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You'd be able to get a positive pregnancy test before you'd get a positive OPK from hCG. Yes OPKs will eventually start reading positive because of hCG but it's a lot later than HPTs will turn positive in the presence of hCG.

Source: lots of research and my own testing. After my MC I was able to turn dark positive HPTs any time of the day for 2 weeks or so. I could even dilute my pee with half water and still get a fairly dark positive HPT. And so I tried with an OPK (ya know, for science) and the OPK had one of the lightest lines I've ever seen on an OPK. I was only about CD 9 at the time so of course an OPK would be negative that early but still. I've concluded that you'd have to have a lot of hCG present to be able to turn an OPK positive with hCG.

It sounds more likely to me that you're ovulating later than normal.

I never knew you could do this. I thought that HPTs detected hcg and OPKs detected LH so two very different tests for different things?

@Lucidity hCG and LH are, at a molecular level, nearly identical. hCG has a beta subunit so it is often said as a means of simple explanation that "hCG looks like LH wearing a funny little hat." Because pregnancy tests need to see the beta subunit they cannot be used to detect hCG. OPKs that test for LH will sometimes read positive if hCG is detected however because the OPK is looking for the basic LH structure and the added subunit doesn't necessarily change the reading.

It isn't uncommon for an OPK to require 100miu to turn positive. Most HPTs only require 25miu of hCG to turn positive. So basically it is theoretically possible for an OPK to turn positive during pregnancy but it would be much later on. It is theoretically impossible for an HPT to turn positive during ovulation since it needs to detect the beta subunit also.

OPKs should not be used as HPTs however.

Is it weird if u get dark postive opk for 4 days?