A survey conducted by the Medical School in Washington, DC, in St. Louis revealed this often domestic pregnancy tests give a false negative result to women who are pregnant, a finding that led the FDA (US Food and Drug Administration) to change its standards to evaluate new pregnancy tests. However, previous tests with a false negative problem are still on the market.
The research led by Ann Grosski, a professor of pathology and immunology and obstetrics and gynecology and a medical director of laboratory services at the Barnes Hospital, in the last decade to 5% of the tests gave a false negative reading. The finding contradicts many advertisements from leading home pregnancy tests that claim to have efficacy results greater than 99 percent.
Producers of pregnancy tests warn that tests performed in the first or second week after conception may be incorrect, as pregnancy hormones may not have increased enough to be detected. But, Gronowski discovered it Pregnancy tests may also give incorrect results in women with a pregnancy of five weeks or more when hormone levels tend to be very high. The specialist published the first article that described the problem in 2009, and since then she continued studying and raising the alarm for this serious but little-recognized failure. Recently, Gronowski and his team published an inscription in the magazine Clinical chemistry in which they evaluated the likelihood that several pregnancy devices give false negative results.
The team examined 11 of the most commonly used pregnancy tests in hospitals similar to those purchased at the pharmacy. From the analysis of the evidence, it turned out that nine were susceptible to false negatives when the levels of the hormone of pregnancy were high.
Pregnancy tests detect the presence of a hormone called human chorionic gonadotrophin (hCG) in the urine, which the body produces only when the woman is pregnant. Hormone human chorionic gonadotrophin is released when the fertilized egg is attributed to the uterine wall, that is when the onset of pregnancy begins.
HCG is a really big molecule composed of two parts: the alpha part and the beta part. To detect both parts of hCG, the pregnancy test introduces the test strip with two different antibodies: the first antibody approaches the alpha unit and the second antibody is directed to the beta unit. Once hCG is properly walled, the release of the famous "second line of hair" is triggered, which indicates that the woman is pregnant. The first line was always present, it's a control test that simply ensures that the antibodies and color work.
Now, there are variations of hCG and the relative amounts of these different variations change during pregnancy, and this interferes with antibody pregnancy tests, which are good for measuring intact hCG.
What happens at times is that the degraded form of the hormone can be found in the urine, which means that in some devices the first antibody binds to the degraded form. The amount of degraded form, called hCG basic fragment, increases with the advancement of pregnancy. The greater the amount of fragmented hormone around it, the more likely the first antibody will accidentally catch the fragment instead of the intact hormone.
However, the signaling antibody does not respond to the fragment, so it does not change the color when it occurs, therefore it receives a negative result even though the hormone may be present.
About how honor this problem, Gronowski stressed that "depends on the device". "Everyone uses different antibodies, and some are better at discriminating the entire hormone of the fragment than others." We analyzed 11 of the most commonly used hospital tests in pregnancy to see if they are prone to false negatives when levels of hormone fragments are high. they were slightly susceptible, two were very susceptible and only two doughs were not susceptible, the worst gave false negatives in 5% of the urinary samples of the analyzed pregnant women. "
Does dilution of urine in water reduce the error? "What's interesting is that some women have discovered our research and posted online pregnancy forums, saying that if you think you are pregnant, but the test is negative, dilute your urine with water and try again, because sometimes the tests are incorrect , and it really can work. Dilution reduces levels of the hormone fragment enough so that the first antibody is more likely to detect the intact hormone again. But I do not recommend it. The best test for detecting pregnancy is a blood test. If there are any doubts, I would recommend that women talk to their doctors and ask for blood tests. "