Ovulation Predictor Kits (OPKs) can help you to anticipate imminent ovulation. They work by measuring the amount of luteinising hormone (LH) that you are excreting in your urine. Typically,you use one test strip a day.
Unlike Pregnancy tests, two lines do not automatically mean the result is positive. A positive result is only when the second test is at least as dark as, or darker than, the reference line. This is because you produce small amounts of LH in the run up to ovulation, but it is the surge in LH that indicates ovulation is likely to occur. To eliminate any confusion or doubt over whether a result is positive or not, some manufacturers make OPK’s that display a smiley face when an LH surge is detected.
Positive results indicate likely ovulation within the next 12-36 hours, so you are entering your most fertile time. The LH surge then passes and OPKs then return negative results again.
Occasionally, an LH surge can occur without ovulation, so it is useful to use other fertility indicators to get a clearer picture of when and if ovulation is occurring within your cycles.
OPKs as Home Pregnancy Tests (HPTs)
LH is very similar in its structure to the pregnancy hormone Human Chorionic Gonadotrophin (HCG). Although very similar in their structure, they are not identical.
Imagine you have twins, and one of those twins is wearing a hat. HPTs look for the identifiers that make the twin, but also the hat. Without the hat, the HPT will not show up positive, even though all the other identifiers are present, whereas the OPK will show positive, whether the twin wears a hat or not.
Because they are not designed to be specific, OPK’s can also show positive results in the presence of HCG. However, they are not designed for this purpose, and a ‘positive’ pregnancy result on an OPK can take much longer to show up than on an HPT which has been specifically designed for the purpose. HPTs cannot be used as an OPK, however.
By Lorraine Berry – Midwife.