Manufacturers of home pregnancy tests usually indicate in the annotation that in the first two weeks after conception, the results may be inaccurate, since at this time a sufficient concentration of pregnancy hormones in the urine has not yet been reached. However, Ann Gronowski, a professor of obstetrics and gynecology at the Barnes-Jewish Hospital at the University of Washington School of Medicine (St. Louis, USA) and her colleagues ten years ago, accidentally discovered that rapid pregnancy tests can produce false negative results for a period of five weeks or more, when pregnancy hormone levels are already very high.
Doctors published the first article on this topic in 2009, and since then they continue to sound the alarm about the inaccuracy of such a popular method of self-determination of pregnancy in the early stages. According to their estimates, up to 5% of rapid tests of various brands sold in American pharmacies show that a woman is not pregnant, when in fact it is not.
In an interview published on the website of the University of Washington School of Medicine, Hronovsky explained why the tests might show erroneous results. The fact is that they all reveal the presence of human chorionic gonadotropin (hCG) in the urine — a hormone that begins to be produced on the sixth or eighth day after fertilization of the egg, after a successful implantation of the embryo into the uterine mucosa.
The level of hCG reaches its peak by the seventh-eleventh week of pregnancy. In the test strip, which is dipped into the urine, there are two different substances (antibodies), one of which reacts directly to hCG, and the second acts as a signal, changing color — thanks to him you see the second blue or pink line on the test saying about a positive result.
However, in the urine, in addition to intact hCG molecules, its partially degraded (decomposed) fragments are also present, the level of which increases with the progression of pregnancy. In some tests, the first antibody mistakenly binds not to hCG, but to its fragments, and the more of them, the higher the probability of error.
The signaling antibody does not respond to the degraded form of hCG, and the second line on the test strip does not appear. Thus, despite the presence of pregnancy, the test shows a negative result.
According to Gronovsky, during the most recent experiment, they and colleagues tested 11 of the most popular rapid tests commonly used in hospitals to see how susceptible they are to fake-negative results in a situation where the concentration of hCG fragments in the urine is high. As a result, seven of them showed incorrect results from time to time, and two — very often. Only two tests turned out to be absolutely accurate. The most inaccurate test did not show the second strip in contact with the urine samples of pregnant women in five percent of cases.
“Unfortunately, it was just the test that we usually used when taking primary patients. After that, we switched to a brand that does not have such false negative problems, ”Gronovsky said. Due to the inaccuracy of the rapid urine tests, Gronovski recommends that women who suspect that they are pregnant should consult a doctor and determine if they are pregnant by a blood test.
See also: "Classic" signs of pregnancy, which may be symptoms of something else.