OPK (Ovulation Predictor Kits)

OPK (ovulation predictor kits)

OPK - Ovulation Predictor KitOvulation Predictor Kits or OPK have become very popular over recent years, as a simple, home-based method to anticipate ovulation and predict a woman’s fertile time each cycle.

There are two standard types of OPK: those that test urine and those that test saliva.

So how do these ovulation predictor kits work, how reliable are they and what additional benefits can charting temperature and cervical fluid offer?


OPK – detecting ovulation

In order to understand how these kits work, it is important to first understand a little about ovulation.  Ovulation, put simply, is the release of an egg from one of your ovaries each cycle.  Ironically, the day your egg is released marks the end of your fertile time, with your best chance of conception resulting from sex in the few days before ovulation.  The day of ovulation each cycle varies from woman to woman.  Even within an individual woman, this day can vary between cycles, especially if your cycles are irregular.  The purpose of an OPK is to identify your fertile days before ovulation and they work by detecting certain hormonal changes that occur over this time.

At the beginning of the menstrual cycle about 15-20 eggs start to mature in each ovary.  Each egg is encased in its own ‘sac’ or follicle and as they develop, the follicles produce the hormone estrogen.  One follicle ultimately dominates and continues to mature while the rest of the follicles start to disintegrate.  Levels of estrogen from the dominant follicle rise dramatically in the days before ovulation until they reach a critical level.  This high level of estrogen triggers a surge of another hormone called Luteinising Hormone (LH).  It is this brief but dramatic rise in LH that triggers ovulation, causing the egg to be released from the ovary.  It is the increased levels of these hormones, estrogen and LH that an OPK is designed to detect.


OPK – how do they work?

Urine based Ovulation Predictor Kits

A urine based OPK detects the increase or surge in LH that occurs just before ovulation.  Depending on the type of urine-based kit you use, you will need to either collect your urine in a cup, or hold a stick in your urine stream.  Coloured bands (or lines) will appear on the stick or test strip to let you know whether or not the LH surge has started.

It is important to follow the manufacturer’s instructions closely and avoid using the test the same way as you would a pregnancy test.  Remember, a test band lighter in colour than the control band is not a positive result – unlike a pregnancy test.  The result is only positive if the test band is equal or darker in colour than the control band.   Likewise, it is not advisable to use your first morning urine sample as you can with a pregnancy test.  This is because LH is synthesized in your body early in the morning and may not show up in your urine until later in the day.  The best time to test is around 2pm.

Once a positive result is detected, ovulation will usually occur within the next 24 – 36 hours and intercourse should take place during this time.


Urine-based OPKs are generally highly accurate in detecting the LH surge, though interpretation of results is not always easy.  Using these kits can be helpful if you are trying to get pregnant, but they are not foolproof.  For information on some of their limitations and how this can be overcome, see below.

Saliva based Ovulation Predictor Kits

A saliva based OPK detects the increased estrogen levels that occur before you ovulate.  As your estrogen levels rise due to the developing follicles in your ovaries, so too does the salt content of your saliva.  When this saliva is put on a slide to dry, the salt crystallizes to form a pattern that looks like ferns.  This ‘ferning’ pattern can be observed with a special ovulation microscope. 

A saliva-based OPK is basically just a small, transportable microscope that typically looks like a lipstick case. Saliva (without bubbles) is placed on a slide with a clean finger or by licking the slide.  It is important that this is done before eating, drinking or brushing teeth.  Once dry, the saliva is observed through the microscope to see if there is a ferning pattern and compared with examples on the manufacturer’s instructions.  If the result is positive, you are likely to ovulate within 72 hours and intercourse should take place during this time. 


Research has found a high correlation between estrogen levels and crystallization of saliva and manufacturers claim 98% accuracy for the saliva-based OPK.  However, although they are quick, easy to use and relatively inexpensive, the saliva OPK don’t appear to work for everyone and interpreting the slides can be difficult.  Although they can give more advanced warning of impending ovulation than the urine-based Ovulation Predictor Kits, they share some of their limitations as discussed below.

OPK – disadvantages and how fertility charting can help

Although OPK can be quite useful when you are trying to get pregnant, they do have their downsides.  The bullet points below, outline some of these disadvantages and how charting cervical fluid and temperature, either alone or in conjunction with OPKs, can help overcome some of these problems.

  • Although both urine and saliva kits test for increased hormone levels prior to ovulation, they do not confirm that ovulation has actually occurred.   An LH surge can occur with or without the release of an egg and likewise increased estrogen levels do not always result in ovulation.  Charting basal body temperature is a simple means of confirming whether or not ovulation did in fact occur. 


  • Because the LH surge is extremely brief, it can be easy to miss.  For this reason, some women may need to test their urine twice a day.  Charting temperature will help you determine if you are just missing the LH surge, or actually not ovulating at all (which would require fertility investigation).


  • A woman can experience ‘false’ LH surges before the real one occurs.  This can result in timing intercourse too early and missing the true fertile window. Women with PCOS may continually produce false LH surges.  Temperature charting will confirm whether or not ovulation has followed an LH surge and whether or not you need to continue testing.


  •  Cervical fluid is critical for sperm survival and transport and having sex on the day you produce your best quality cervical fluid optimizes your chances of conception.   Conversely, a complete absence of fertile cervical fluid can be a cause of subfertility.  Ovulation predictor kits give no information about cervical fluid so without charting this valuable symptom you may miss clues about your true fertility status.


  • A woman’s fertile time is a maximum of 6 days, each cycle, ending on the day of ovulation.  However, because ovulation can occur as early as 12 hours after the LH surge is detected, much of this fertile time can be missed.  Charting cervical fluid will give you ‘advanced warning’ of impending ovulation and more time to plan baby-making sex.


  • By the time the LH surge is detected, your cervical fluid may have already started to dry up (meaning less chance of sperm survival).  Charting cervical fluid will allow you to recognize your most fertile day or days, which may come before the LH surge takes place.


  • For women with irregular cycles, it can be a challenge to know when to begin testing with saliva or urine-based kits.  Because the day of ovulation can vary widely between cycles and a typical urine-based kit only contains 5 – 9 days worth of tests, one kit is sometimes not enough and it can become very expensive.  Charting cervical fluid will let you know the appropriate time to begin testing.


The Fertility Focus service specializes in teaching fertility awareness and how to chart cervical fluid and temperature to maximise your chances of getting pregnant.  For more information please browse the website further or contact us now.

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