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The OPK uses a urine test that measures luteinising hormone (LH) levels. The levels of LH in urine are usually low but will increase sharply for a 24-48 hour period just before ovulation. OPKs can help you pinpoint when you ovulate more precisely than tracking your basal body temperature or cervical mucus, particularly if you have an irregular cycle. Ovulation typically occurs about half-way into your menstrual cycle (about 12-14 days before your period on average). You'll know you're a few days away from ovulation when you begin to see watery discharge resembling egg whites. When you begin to see this discharge, begin using the OPK. Because a kit will only contain a limited number of testing strips, it's important that you wait until this point before beginning. Otherwise, you may go through all of the strips before you actually begin ovulating. Follow the instructions provided with the kit. You should be careful to test your urine at the same time each day. Avoid being either under- or over-hydrated, as it may artificially raise or lower LH levels. Many OPKs use a urine stick or strip to measure your LH levels and will indicate your results using colored lines. A line close to the color of the control line typically indicates elevated LH levels, meaning there's a good chance you're ovulating. A line lighter in color than the control line typically means you're not yet ovulating. If you use OPKs several times without any positive results, consider seeing an infertility specialist for consultation to rule out infertility issues. Though the test is usually accurate, you may miss your ovulation window if you don't time the testing correctly. For that reason, OPKs are best used in conjunction with another ovulation-tracking method, like tracking basal body temperature or cervical mucus, so you have a better sense of when to begin taking the urine tests.

Summary:
Buy an Ovulation Predictor Kit (OPK), available at most drug stores. Pay attention to your menstrual cycle. Begin testing your urine each day. Know what your results mean. Know the limitations of using an OPK.