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Study on HPT accuracy...I thought some of you might find this interesting...

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Cake from Outer Space!

Member since 5/05

14007 total posts


Study on HPT accuracy...I thought some of you might find this interesting...

How do home pregnancy tests work?
Home pregnancy tests measure the presence of a telltale hormone in your urine called human chorionic gonadotropin (hCG). This hormone, produced by cells from the placenta, first gets into your bloodstream when the fertilized egg implants in your uterus (about six days after fertilization). The amount of hCG in your body then increases rapidly over the next few weeks, doubling in amount about every two days. By 14 days after fertilization (about the time you would expect your period), some home pregnancy tests may be able to detect the hormone in your urine and give you a positive result. But in spite of their claims, most are not sensitive enough to guarantee you an accurate result at this point. In fact, you're much more likely to get an accurate result if you wait until a week after your expected period before testing.

How accurate are they on the day you miss your period?
Most home pregnancy tests claim to be "greater than 99 percent accurate" and imply that you can use them as early as the day you miss your period, but a study published in 2004 in the American Journal of Obstetrics and Gynecology has shown that this is misleading. The fact is that the amount of hCG in the urine on any given day after implantation can vary a great deal from one woman to another. Laurence Cole and other researchers at the University of New Mexico evaluated 18 currently available tests and found that only one was consistently sensitive enough to detect the levels of hCG that most pregnant women were likely to have on the first day of their missed menstrual period. Most of the other tests were only sensitive enough to pick up about 16 percent of pregnancies at that point, though many were likely to be accurate a week after an expected period was due.

So how can all these tests claim to be accurate so early in pregnancy?
They don't. They only claim to be "greater than 99 percent accurate" in general, and then separately, they suggest that you can use them as early as the day you miss your period. According to the current Food and Drug Administration (FDA) regulations, in order to claim that a home pregnancy test is "greater than 99 percent accurate," the manufacturer only needs to demonstrate that the test performs as well in the lab as an existing test more than 99 percent of the time. Since today's home pregnancy tests are more sensitive than previous products, it's not surprising that they are able to maintain this claim, but it has nothing to do with a test's ability to detect pregnancy at the time of a missed period.

How can I tell which tests are the most sensitive?
It's not easy. In the New Mexico study, First Response Early Result was the most sensitive test the researchers checked, though the test showed only a faintly positive result for the lowest levels of the hormone. But new products come out all the time and other brands may make improvements to their products at any time. Consumer Reports ranks 18 available tests on its Web site (though you have to subscribe to see the report).

Some package inserts provide some information about a test's sensitivity — that is, they report the lowest concentration of hCG (in milli-International Units per milliliter of urine) that the test can detect. For example, a pregnancy test that claims to be able to detect hCG at 20 mIU/ml should theoretically be more sensitive than one that claims to detect it at 50 mIU/ml. Unfortunately, this information is often misleading because there are actually different kinds of hCG and these numbers won't necessarily tell you how good a test is at detecting the kind that's most relevant in early pregnancy.

How do I use a home pregnancy test?
First check the expiration date on the test and make sure it's still valid, especially if you've had it around for a while. If you've been storing the test anywhere that gets moist or warm (like the bathroom), it may have deteriorated, so it's best to throw it away and get a new one. When you're ready to test, read the directions carefully because they'll vary with different brands. Some require you to urinate in a cup and then, using a supplied dropper, place a small sample in a testing well. Others let you pee directly onto a stick. (Some will let you do either.)

The tests also vary in how they display results: Some show pink or blue lines on the test strip, while others reveal a red plus or minus sign in a window. Most have a control indicator (often a second line or symbol) that's supposed to indicate whether the test is valid. If the control indicator doesn't show up properly, the test may be faulty. If this happens to you, you can usually call the manufacturer and have them send you a new one, though it might not arrive soon enough for you to use it that month. Most tests say that you can check results in about five minutes, although the New Mexico study found that in some cases, you may have to wait ten minutes for a positive result.

If the test shows a negative result, wait another few days or a week and try again if you still haven't gotten your period. (If you ovulated later in your cycle than you thought, you may have taken the test too early to get a positive result.) For best results, try taking the test first thing in the morning, when your urine is most concentrated.

Whatever you do, don't assume that one negative result means you're not pregnant. If you don't get your period as expected, remember that you might still be pregnant (it's no time to go off on a drinking binge or do other things that are unsafe in pregnancy). If you still haven't gotten your period or a positive result in another week or so, make an appointment with your practitioner so you can try to find out why.

Is it possible to get a false positive result?
False positives — when the test says you're pregnant but you're not — do happen. The New Mexico study found two brands that gave occasional false positive results. If you get an early positive result and then get your period soon after, you may also have had what's called a "chemical pregnancy." That means a fertilized egg implanted in your uterus and developed just enough to start producing hCG but then stopped developing for some reason. This happens with about 30 to 50 percent of all fertilized eggs because they're abnormal or otherwise incapable of developing into and surviving as an embryo. If this is the case, you'll go on to get your period (though it may be a little heavier and a few days later than usual). When pregnancy tests were less sensitive than they are today, these so-called "chemical pregnancies" were never identified. Many practitioners think that's another good reason to wait until a week after your period is due to perform a home pregnancy test.

Note: An ectopic pregnancy can give you either a positive or negative result on a pregnancy test. Call your practitioner right away if you have any abdominal pain or abnormal bleeding, no matter what a pregnancy test tells you.

How are home pregnancy tests different from ones performed in doctors' and midwives' offices?
Many clinics use the very same kind of urine pregnancy tests that are marketed for home use. Doctors and midwives generally only use blood tests to test for pregnancy when they need to know exactly how much hCG is in your blood or what's happening to the level over time — if they think you might be miscarrying, for example.

Where can I buy a home pregnancy test?
You can buy a home pregnancy test without a prescription at most drugstores and on the Web. They generally cost between $8 and $18, but note that some packages contain more than one test, making them a better bargain

Posted 9/7/05 2:12 PM
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Re: Study on HPT accuracy...I thought some of you might find this interesting... Chat Icon

Posted 9/7/05 2:46 PM

Cake from Outer Space!

Member since 5/05

14007 total posts


Re: Study on HPT accuracy...I thought some of you might find this interesting...

Tee hee...I'm not really that bright!

Posted 9/7/05 3:06 PM

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