Accuracy of Home Pregnancy Tests

Home Pregnancy Tests. Photo Courtesy of PriceGrabber

A home pregnancy test detects human chorionic gonadotropin (hCG), the pregnancy hormone. Although there are many different types and brands of home pregnancy tests, they all function in basically the same manner. These tests will check the urine to try to detect the presence of the hCG hormone.

Available Home Pregnancy Tests

There are several varieties of home pregnancy tests. In general, they all operate in a similar way. Most pregnancy tests will use a dipstick to collect and analyze the urine. These one-step kits are usually considered the most convenient to use; most sticks can be briefly held in the urine stream or dipped into a collection cup.

There are also other home pregnancy tests which require a woman to mix a small amount of her urine with a special liquid or powder. Even though each test may function in the same manner, it is still important to read the test’s instructions as these could vary between each pregnancy test brand.

How Home Pregnancy Tests Work

These tests measure the amount of the pregnancy hormone, hCG, found in a woman’s urine. The female body will only release hCG when she is pregnant (when a fertilized egg implants in the uterus). In most women (but not all), this occurs about 6 days after conception. The hCG levels increase with each passing day of a pregnancy, doubling about every 2 days.

Home pregnancy tests can reliably detect this hormone approximately one week after a missed period. Although some home tests could detect hCG as early as a missed period, the majority of these are not sensitive enough to guarantee the results if taken this soon.

Accuracy of a Pregnancy Test

Accuracy claims can be somewhat misleading. Many home pregnancy tests typically maintain a 99 percent accuracy rate or better. The problem lies in the fact that these tests also imply that this accuracy could be expected if you take the test as early as the day of a missed period.

A 2004 study published in The American Journal of Obstetrics and Gynecology by Dr. Laurence Cole and researchers at the University of New Mexico confirmed the deceptive claims of many of these early pregnancy tests. Cole explained that these claims are misleading due to the high degree of variation in the amount of hCG occurring in the urine on any given day after implantation occurs. Researchers found that for early detection of pregnancy, most tests were not sensitive enough to detect hCG on the first or second day after a missed period.

In fact, out of the 18 brands tested, only one, the First Response, Early Result Test, was sensitive enough to "consistently detect 12.5 mIU (milli-International Units per milliliter of urine) of hCG, considering the manufacturer's suggested read time, and produced both clear and faintly discernible positive results" on the first and second day after a missed period. This level of sensitivity (12.5 mIU) is required to detect 95 percent of pregnancies at the time of a missed period.

According to Dr. Cole, et al., "three brands gave some form of a positive result, whether clear or faintly discernible at the suggested reading time using the 25 mIU concentration of hCG (Clear Blue Easy, One Minute; Clear Plan Easy; First Response, Early Results)." This level of sensitivity is able to detect 80 percent of pregnancies on the first or second day of a missed period. Most of the other tests were only able to detect hCG in 16 percent of pregnancies when tested a day or two after a missed period.

Determining the Sensitivity of a Home Pregnancy Test

Usually, the more sensitive the test, the earlier you can get an accurate pregnancy test result. Consumer Reports experts advise when buying a home pregnancy test, "the most sensitive tests currently detect about 15 to 25 mIU of hCG, corresponding to detection of pregnancy within about a day of a missed period for 90% of women."

For most women, the more sensitive the test is, the more accurate the result; however, if a woman has hCG in her system from a recent birth, miscarriage or fertility drugs, a less-sensitive test may be a better option.

When trying to determine how sensitive a test is, you can check the package insert. Most should explain the lowest mIU concentration of hCG that the test can detect. In theory, a pregnancy test that maintains that it could identify hCG at 25 mIU should be more sensitive than one that can identify this hormone at 40 mIU. The only thing to be aware of is that a woman produces different kinds of hCG during a pregnancy, so sometimes the sensitivity claims of pregnancy tests do not actually indicate that the test will pick up on the type of hCG most associated with an early pregnancy.

The Reason Why Early Home Pregnancy Tests Can Make These False Claims

These misleading claims tend to be somewhat of an advertising gimmick. The FDA regulates that a home pregnancy test can maintain greater than 99 percent accuracy as long as the manufacturer shows that at least 99 percent of the time, in a lab, their test functions as well an existing test. The home pregnancy tests currently available are, in fact, more sensitive than earlier tests, so the companies can maintain these claims.

The "catch" is that these manufacturers make the accuracy claims in a general sense; they then suggest (separately) that a woman could use the test as early as the day of a missed period. However, the lab results typically do not reflect the test's ability to detect pregnancy this early on.

When to Take a Home Pregnancy Test

It is important to point out that the 90-99 percent test accuracy claims are generally true once a woman is further along in a pregnancy – just not during the first few days. This is why it is usually best to wait at least one week after a missed period to take a pregnancy test. Keep in mind, that even though many home pregnancy tests can be taken as early as the first day of a missed period (and claim to be 99 percent effective on the day of a missed period), most of these pregnancy tests will not consistently detect a pregnancy this early.

Pregnancy Test Results

Depending on the design of the test, the pregnancy test results may be simpler or harder to read. A test that has enough contrast between the line (or symbol) and the background makes the results easier to interpret. Some brands indicate that an evaporation line may appear if the test is left to sit past a certain time frame; this line may make it more difficult to accurately interpret the test results

Negative Test Results

A home pregnancy test is more likely to yield a false–negative result (meaning, that you really are pregnant) rather than a false positive result (the test says you're pregnant when you are not).

A false-negative test result can occur if:

  • Your urine is diluted - Many tests suggest that you perform the test in the morning, right after you wake up. This is because your urine is usually the most concentrated at this time. If you drink too much liquid before performing the test, you may end up with an inaccurate result as well.
  • You have performed the test too soon - In order to at least have the possibility of an accurate result, you should wait at least a day after your missed period (because a home pregnancy test cannot detect a pregnancy any sooner than that). However, to decrease the chances of a false-negative result, it is better to wait seven days after your period was due.
  • You have timed the test incorrectly - It is important that you perform the pregnancy test within 15 minutes after collecting a urine sample. Make sure that you follow the test’s instructions as to how long it takes to analyze the results. If you check the results too soon (before the prescribed time period), the test result may appear to be negative.

    Even if you receive a negative result, if your period has not started within a week after a taking the test, you should take another pregnancy test. At this time, if you still have not gotten your period or a positive result, it is a good idea to make an appointment with your health-care professional to determine what is going on as circumstances like stress, excessive exercise, illness and hormonal imbalances can also cause a woman to miss a period. Your doctor can help you get your menstrual cycle back on track.

    Positive Test Results

    Typically, if you receive a positive result (even if it is very faint), this indicates that you are pregnant. It is possible to obtain a false-positive result (the test is positive, but you are really not pregnant) – though this happens very rarely.

    You may end up with a false-positive if:

    • You are taking a prescription medication that contains hCG, such as Pregnyl, A.P.L., Profasi, Chorex, Novarel, or Ovidrel or the medication promethazine (used as an antihistamine in a combination cough and cold products to treat allergy symptoms and to treat nausea or vomiting from illness or motion sickness).
    • You have traces of blood or protein in your urine.
    • You have used an expired or damaged pregnancy test.
    • You are currently taking diuretics.
    • You had what is known as a chemical pregnancy. This means that a fertilized egg did implant into your uterus and developed just enough to trigger the production of hCG, but then, for whatever reason, stopped developing. Typically, about 30 to 50 percent of all fertilized eggs end up as chemical pregnancies because of abnormalities or other reasons that make further development impossible. When this occurs, most women will end up getting a period (though it may be a few days later or heavier than usual). The higher likelihood of a chemical pregnancy to occur is another reason why it may be better to wait at least a week after your period is due to take a home pregnancy test.

      Keep in mind that a pregnancy test with a flawed design may also yield a false-positive result. According to Dr. Cole, et al., two of the 18 pregnancy test brands tested had unmistakable technical or design problems. "These tests both gave falsely positive hCG test results with urine containing no hCG and also gave numerous invalid results as indicated by the absence of a confirmation or validity line." Pregnancy tests that do not work correctly "may generate false hope or great confusion among users."

      Where to Get a Pregnancy Test

      Most grocery stores, drug stores, and websites sell home pregnancy tests over the counter (without needing a prescription). Depending on the brand and how many tests come in the box, tests can cost between $4 and $20. Read the package carefully as some may contain 2 tests, so these could be a better deal. If you think you may need a second test because you have irregular periods or if you are testing right after a missed period, it is usually a better bargain to purchase a 2-pack than paying separately for another test.

      Blood Pregnancy Tests Versus Home Pregnancy Tests

      The urine pregnancy tests performed at most doctor’s offices are basically the same kind as the ones found over the counter. The main difference in pregnancy testing is that some healthcare professionals will use blood pregnancy tests, which can detect a pregnancy much earlier than urine tests can. Another advantage of a quantitative blood test is that it can reveal the exact amount of hCG in the blood. This is helpful to assess how far into a pregnancy a woman may be or if there is the possibility that a woman may be miscarrying.

      What to Do Next

      If you receive a positive result on a home pregnancy test, you should make an appointment to see your health-care professional. You should also see your doctor if you have taken a few home pregnancy tests and have received mixed results. Your health-care professional may perform a blood test or pelvic exam to confirm your positive pregnancy result. The sooner you know whether or not you are pregnant, the sooner you can start to make decisions about your pregnancy.

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      Article Sources

      • Cole, L., et al. (2004). “Accuracy of home pregnancy tests at the time of missed menses”. American Journal of Obstetrics and Gynecology, Vol. 190(1), 100-105.
      • Consumer Reports. (2006). “Pregnancy Tests Reviews”.