Ovulation tests (OPKs) are excellent tools to help detect potential fertile days, but they do not confirm the occurrence of ovulation. Most ovulation tests available on the market measure the LH level, which increases in a woman’s body before ovulation (this hormone is responsible for the release of the egg cell from the Graafian follicle). An ovulation test will indicate a positive result when the level of LH increases dramatically and reaches its maximum value. Then, about one day after this peak value has been detected (called "LH surge"), ovulation occurs.

Sometimes, however, even though the body has prepared itself for ovulation and LH has been released (the test indicates a positive result), ovulation will not occur or will be postponed. Because of this, it is recommended to continue intercourse up to the moment when your temperature increases and ovulation is indicated on your chart.

At other times, an ovulation test can also come back negative, even though ovulation has taken place. This may happen when the period of LH surge in your body is too short to be detected by a daily test. For example, you might do the test early in the morning, but the LH level starts to increase and reaches its peak value hours later. The next morning, when you do another test, the LH level will be past its peak value and will once again indicate a negative result, even though the LH surge has occurred and so ovulation will probably take place soon. To minimize the risk of missing your LH peak, which usually starts to increase early in the morning and may not be detected in the first morning urine, try to do ovulation tests at around noon.

Additionally, always take your basal body temperature. If you take your temperature correctly, the results should never mislead you (Is temperature a reliable indicator of ovulation?)