Ovulation tests read LH, not ovulation itself, so they fail when LH is high or disturbed for other reasons. The main case is PCOS, which often gives chronic false positives. Tests are also unreliable in perimenopause, after recent clomiphene or letrozole, after a recent pregnancy and in hormonally stimulated cycles.
Ovulation tests measure the level of LH (luteinizing hormone) in urine. In a typical cycle, LH rises sharply in a surge that triggers ovulation within 24 to 36 hours, so a positive is a useful signal. But the test only sees the hormone: it cannot tell whether ovulation actually follows.
That is why tests are one of the most common sources of frustration for TTC women: many use them the wrong way, at the wrong times, or expect them to do something they don't do.
Women with Polycystic Ovary Syndrome (PCOS) frequently have chronically elevated baseline LH. That means ovulation tests can read positive several times in a cycle without any real ovulation happening.
For PCOS, the book is clear: basal body temperature plus cervical mucus are more reliable than LH tests. Temperature confirms after the fact that ovulation really happened, and mucus shows the window opening.
PCOS is the most common diagnosis in women of reproductive age, affecting roughly 8 to 13% of them, and one of its defining criteria is irregular cycles or absent ovulation. In blood work, the LH/FSH ratio in PCOS is frequently above 2. That is the setting in which a positive strip proves the least.
In these situations, the book treats the tests as unreliable. Cervical mucus and basal temperature, covered in the chapters just before, don't depend on reading LH in urine.
When a positive is in doubt, temperature answers after the fact: a sustained rise of 0.3°C or more, held for 3 consecutive days, confirms that ovulation happened. A chart with no biphasic shift at all is one of the signs of a cycle without ovulation.
An occasional anovulatory cycle is normal, even in healthy women. Several in a row deserve medical investigation.
These limits are part of Chapter 17 of Map to the Positive, in Station II (Attunement), alongside the start day by cycle length, the six mistakes that invalidate a test and an honest comparison of test types. Station II also teaches the signs that work when LH tests don't: cervical mucus and basal temperature. Station III (Optimization) returns to PCOS with myo-inositol in its supplementation chapter. Station IV (Signs) covers PCOS among the 8 most common diagnoses.
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