Ovulation Predictor Kits (OPK's) are an essential tool that many women, who are trying to conceive, choose to use to help determine when the best time for conception is. 

An OPK detects a surge in the Luteinizing hormone (LH) that occurs when ovulation is about to take place. The pituitary gland releases LH, and the luteinizing hormone is always in the bloodstream in small amounts. Its job is to stimulate the ovaries into producing and releasing eggs. The LH indicates that ovulation will happen in about 24 to 36 hours. The day the LH surge is detected and the day after are the most likely time for conception to occur.

Clomid is the trade name of the medication Clomiphene citrate. Clomid causes the hypothalamus and pituitary gland, located in your brain, to release hormones called GnRH (gonadotropin-releasing hormone), FSH (follicle-stimulating hormone) and LH (luteinizing hormone).  FSH and LH, in turn, stimulate the ovary. 

Clomid dosages range from 59 mg to 200 mg, and patients take it for 5-9 days.  Clomid is among the most inexpensive of available fertility drugs.  It's easy to take (orally rather than by injection), and it is the first line drug used for ovulation induction in patients with PCOS and other ovulatory disorders.

However, Clomid is a potent and complicated medication.  Clomid reacts with all of the tissues in the body that have estrogen receptors, such as the hypothalamus, pituitary, ovary, endometrium, vagina, and cervix.  

Clomid influences the way that the four hormones required for ovulation, GnRH, FSH, LH, and estradiol work together in the body.  While we do not entirely understand the mechanisms by which this medication works, it appears that Clomid fools the body into believing that the estrogen level is low. This altered feedback information causes the hypothalamus (an area of the brain) to make and release more gonadotropin-releasing hormone (GnRH) which in turn causes the pituitary to produce and release more FSH and LH.  More follicle stimulating hormone and more luteinizing hormone should result in ovulation - the release of one or more mature eggs.

It is possible for Clomid to cause an OPK to register a false positive result.  A false positive will most likely happen if the ovulation test is used too soon after finishing the prescription for Clomid.  Ovulation test manufacturers recommend that you should wait at least three days after stopping Clomid before using a test.

If you take Clomid on days 3-7 of your cycle, you can begin to use the OPK on day 10.  If you take Clomid on days 5-9, then you should wait until day 12 to use the OPK.  Because Clomid causes the release of LH into the bloodstream, the ovulation predictor test will detect this LH.  If you test too early, the LH that the ovulation test is picking up would not be because of ovulation and would be a false positive result.

Source:

“Fertility Drug Types: Injectables Hormones, Clomid, and More.” WebMD, WebMD, 2017, www.webmd.com/infertility-and-reproduction/guide/fertility-drugs#1.