Ovulation tests detect an increase in the luteinizing hormone (LH). This LH surge causes the ovary to release the egg, which is known as ovulation. Ovulation normally occurs 24 to 48 hours after the LH surge, which is why Flo predicts ovulation for the next day after a positive ovulation test result is logged.
However, it is also possible to receive a negative result and still ovulate. If you have an LH surge, for example, you may take a test before your surge is detectable by your kit and another test when your LH has already begun to decrease and is no longer detectable by the kit.
This is why we don’t rely on negative ovulation tests, because we don’t know if ovulation has already happened or if it’s going to happen.