1. OPKs do not test for ovulation. Ovulation predictor kits measure the LH surge that precedes ovulation, but can't confirm whether you have actually ovulated. Occasionally, an egg fails to emerge from its follicle after the LH surge has occurred, a condition known as LUFS (luteinized unruptured follicle syndrome).
2. OPKs don't indicate whether cervical mucus is conducive to fertilization. The mucus produced by the cervix in midcyle, which you experience as vaginal discharge, must have a certain consistency (clear and elastic, like egg whites) in order to provide a fertile environment for sperm to swim through. OPKs do not monitor cervical mucus.
3. OPKs don't work when certain fertility drugs are present in the system. OPKs do not function reliably when injectable fertility drugs such as Pergonal or the hormone hCG (e.g., Profasi) are present in the system.
4. OPKs do not work consistently on women over 40. Some women in their 40s, especially those approaching menopause, have increased levels of LH in their systems at all times, rendering the tests invalid.
5. OPKs don't come cheap. These kits cost between $15 and $50 depending on the brand and number of tests included. Each is good for a single cycle.