Visit the News HubTest results later in pregnancy may be misleading
by Tamara Bhandari•April 18, 2019
Getty ImagesPregnancy tests can sometimes give a false negative result to women several weeks into their pregnancies, according to research by Ann Gronowski, PhD, a professor of pathology and immunology at Washington University School of Medicine in St. Louis. Her findings led the FDA to change its standards for evaluating new pregnancy tests, but old tests with the false-negative problem are still on the market.
Each year, women in the U.S. rely on some 20 million home pregnancy tests to learn potentially life-altering news. Despite marketing claims that such tests are 99 percent accurate, research at Washington University School of Medicine in St. Louis over the past decade has shown that up to 5 percent of pregnancy tests return results indicating a woman is not pregnant when, in reality, she is.
Gronowski
Makers of pregnancy tests advise that tests taken in the first week or two after conception could be inaccurate because pregnancy hormones may not have risen high enough to be detected. But Ann Gronowski, PhD, a professor of pathology and immunology, and of obstetrics and gynecology, and medical director of core laboratory services at Barnes-Jewish Hospital, discovered that pregnancy tests can also give incorrect results to women five weeks or more into their pregnancies, when hormone levels tend to be very high. She published the first paper describing this problem in 2009, and since then has continued studying and raising the alarm on this serious but under-recognized issue. Recently, she and colleagues published a paper in the journal Clinical Chemistry, in which they evaluated how likely several pregnancy devices were to give false negative results.
Q: How did you discover that some pregnancy tests produce false negative results?
About a decade ago, a woman came to the emergency department saying she was pregnant and was experiencing cramping and spotting – which can be signs of miscarriage – but the pregnancy test we gave her at the hospital came back negative. She was certain she was pregnant, so we performed a blood test and an ultrasound, both of which confirmed she was pregnant. Shortly afterward, we had a similar experience with a second patient, and at about the same time a colleague at Vanderbilt University called me with a similar observation. So we started investigating, and we published a paper describing the problem and its cause.
Q: Why do pregnancy tests sometimes return false negative results?
We use a kind of pregnancy test in the emergency department that is similar to what you can buy at a drugstore. These tests detect a pregnancy hormone called human chorionic gonadotropin (hCG) in a urine sample. One antibody captures the hormone in the urine, and the other serves as a signal. It has a color that becomes visible when it detects the correct hormone. That’s the second blue or pink line you see on a test, and it indicates a positive result (the first line is just there to show the device is working properly).
But a degraded form of the hormone also can be found in the urine, and in some devices the first antibody will bind to the degraded form. The amount of the degraded form, called hCG core fragment, goes up as pregnancy progresses. The more of the fragmented hormone that is around, the more likely the first antibody will accidentally capture the fragment instead of the intact hormone. However, the signal antibody does not respond to the fragment so it does not change color when that happens, and therefore you get a negative result even though the hormone might be present.
Q: How common is this problem?
It depends on the device. They all use different antibodies, and some are better at discriminating the full hormone from the fragment than others. We looked at 11 of the most commonly used hospital pregnancy tests to see if they were susceptible to false negatives when levels of the hormone fragment were high. Seven were somewhat susceptible, two were highly susceptible, and only two tests were not susceptible. The worst one gave false negatives in 5 percent of the urine samples of pregnant women tested. That was, unfortunately, the test we were using when that initial patient came in. Based on our research, we have switched to a test that does not have this false-negative problem.
Q: Are the makers of pregnancy tests doing anything to solve this problem?
Based on our work, the Food and Drug Administration has acknowledged the need for pregnancy tests that are not susceptible to false negatives. The FDA requires that all new tests generate a positive signal even when concentrations of the fragmented hormone are high. Unfortunately, this requirement does not apply to tests that were already FDA-approved.
Q: What can women do if they think they received a false negative on a pregnancy test?
What’s interesting is that some women discovered our research and have been posting in online pregnancy forums, saying that if you think you’re pregnant but the test comes back negative, dilute the urine with water and try again because sometimes the tests are wrong. And that can actually work. Dilution reduces levels of the hormone fragment enough that the first antibody is more likely to detect the intact hormone again. But I do not recommend this. The best test to detect pregnancy is a blood test. If there’s any doubt, I would recommend women talk to their physicians and request blood tests.
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FAQs
Very high levels of hCG
Why do I keep getting false negative pregnancy tests? ›
You might get a false-negative if you: Take the test too early. The earlier you take a home pregnancy test, the harder it is for the test to find HCG . For the most accurate results, take a home pregnancy test after the first day of a missed period.
What are the odds of a false negative home pregnancy test? ›
“The false negative rate, if done appropriately, is less than 1%.” If someone waits until the appropriate point in their menstrual cycle and ensures that their urine is concentrated enough at the time of testing, then it's highly unlikely that an at-home pregnancy test will provide incorrect results, he explains.
Can I be 3 months pregnant and still test negative? ›
A false negative pregnancy test can happen because you conceived later than you think, you didn't take the test correctly, your urine was diluted, or even because you're pregnant with multiples.
What is the most common error in home pregnancy testing? ›
Confusing Evaporation Lines
Many tests show two lines when hCG is detected and one line when hCG isn't detected, or a plus or minus sign. For the line tests, sometimes, a faint-colored second line appears. This may represent an early pregnancy or may simply be an evaporation line.
Can I be 5 weeks pregnant and still test negative? ›
Pregnancy tests can sometimes give a false negative result to women several weeks into their pregnancies, according to research by Ann Gronowski, PhD, a professor of pathology and immunology at Washington University School of Medicine in St.
Can too much pee on a pregnancy test make it negative? ›
False results — either a false negative or a false positive — mainly happen due to using the test incorrectly. The main reason for a false-negative is testing too early. You might also get a false-negative if you use a home test incorrectly, such as using too much or too little pee.
Could I still be pregnant if home pregnancy test says negative? ›
Yes, it is possible. Getting a negative result doesn't mean you're not pregnant, it may just mean your hCG levels are not high enough for the test to detect the hormone in your urine.
Can you be 2 weeks late and test negative? ›
If you happened to ovulate later than expected, or if you did get pregnant closer to your expected period than what is typical, the pregnancy wouldn't be far enough along to show up on a test yet if you tested right around your missed period.
Why do I have cramps but no period and a negative pregnancy test? ›
Why do I have cramps but no period? Your period may be late or, depending on your other symptoms, you could be pregnant or have one of several other conditions, such as pelvic inflammatory disease (PID), endometriosis, uterine fibroid or polyps, or polycystic ovary syndrome (PCOS).
Low hormone levels
If you're trying to get pregnant, there's good news: You may still be pregnant. Sometimes, levels of the pregnancy hormone human chorionic gonadotropin (hCG) early in pregnancy aren't high enough for a home pregnancy test to detect.
Is it possible to be 8 weeks pregnant and have a negative test? ›
Confirming Your Pregnancy at 8 Weeks
It's also possible to receive a negative pregnancy test yet still be pregnant! To know for certain, make an appointment with your healthcare provider. Once you confirm your pregnancy, you might start wondering when your due date is if you're eight weeks pregnant.
Can I still be pregnant if the test is negative and no period for 2 months? ›
It's important to know that false negatives are possible. Most of the time, they can occur when you take a pregnancy test too early in your cycle, or if you don't closely follow the manufacturer's instructions. Fluctuations in your menstrual cycle also could be at the root of a false negative.
What are 3 things that can produce a false negative on a pregnancy test? ›
Home pregnancy tests are very reliable, but false negatives are possible in rare occasions. They might happen when your body hasn't produced enough pregnancy hormone (hCG) for the test to pick up. Other factors—such as user error or an expired test—might also be to blame.
How often are at home pregnancy tests wrong? ›
Home pregnancy tests can be up to 99% accurate. However, in some instances, they may produce a false-positive result. Incorrect test usage, previous abortions and miscarriages, and some medications may lead to a false-positive pregnancy test result. People should always see their doctor after a positive pregnancy test.
Can twins cause a false negative pregnancy test? ›
It's actually possible for twins (or triplets, quadruplets, etc.) to cause a false negative pregnancy test. That's because multiple pregnancies produce elevated levels of hCG, and if they're high enough, it can interfere with the results.
What causes hCG levels to rise if not pregnant? ›
The exact role of hCG outside of pregnancy remains unclear. In premenopausal women, hCG and LH levels rise during ovulation. As women get older, hCG levels, like those of FSH and LH, rise due to loss of negative feedback inhibition from estrogen and progesterone.
How long does it take for hCG to show up in urine? ›
hCG is a hormone produced by your placenta when you are pregnant. It appears shortly after the embryo attaches to the wall of the uterus. If you are pregnant, this hormone increases very rapidly. If you have a 28 day menstrual cycle, you can detect hCG in your urine 12-15 days after ovulation.
What if I have symptoms of pregnancy but the test is negative? ›
Why You May Have Pregnancy Symptoms When the Test Is Negative. A negative test result doesn't always mean you're not pregnant. Medications, underlying conditions, and timing can affect the outcome. Getting a blood test or ultrasound is the most reliable.
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Related: Blood test is highly accurate at identifying Alzheimer’s before symptoms arise