The prospect of any home pregnancy test sold in pharmacies guarantees 99% accuracy. However, scientific research shows different numbers. Up to 5% of all tests return false negatives.
The test measures a hormone in the body called human chorionic gonadotropin (HCG), produced during pregnancy. The manufacturers advise to carry out the test from the first or second week after conception because they consider that it can already be detected in the urine. However, it does not always rise enough to be recognized in a test.
In the United States, the sale exceeds 20 million units per year. Ann Gronowski, obstetrician and medical director of laboratory services at Barnes-Jewish Hospital, has discovered that these inaccuracies can occur even in later stages when hormone levels are higher. Already in 2009, she gave her first warning, but now she published an article in the journal Clinical Chemistry that describes exhaustively what is happening and the probabilities for these devices returning false results.
The first case she had knowledge of was that of a patient who went to the emergency room with cramps and bleeding, which led her to think that she might be suffering from spontaneous abortion. The pregnancy test was negative, so we had to perform a blood test and an ultrasound that confirmed that she was indeed expecting a child. Shortly after, a colleague from Vanderbilt University called her with a similar experience, so she decided to investigate.
Where is the confusion?
The problem, according to Gronowski, is that a degraded form of the hormone, called a fragment of HCG nucleus, can also be detected in the urine, which increases as the pregnancy progresses. “The greater the amount of this fragmented hormone, the more likely it is that the first antibody will accidentally capture it instead of the intact hormone. Of the eleven tests analyzed, the most common in pharmacy, seven had a moderate susceptibility to false negatives; two high and only the other two were not susceptible. The worst of them gave a false negative in 5% of the samples. From this evidence, her advice is that, if there is suspicion of pregnancy and the test is negative, request a blood test.