EXPERTS have warned that some home pregnancy tests could give negative results to women who are in fact pregnant.
Research carried out in the US found that over the past decade up to five per cent of tests gave a false negative reading.
Getty – Contributor
The findings contradict marketing claims by some of the leading pregnancy test companies, which claim to give results that are more than 99 per cent accurate.
A team from the Washington University School of Medicine in Missouri looked at 11 of the most commonly used hospital pregnancy tests, which they say work in the same way as those bought from a pharmacy.
They found that nine of which were susceptible to false negatives when levels of the hormone fragment were high.
The worst one gave false negatives in five per cent of the urine samples of pregnant women tested.
False negatives
Manufacturers advise that tests taken in the first two weeks after conception could be wrong because pregnancy hormones might not have risen enough to be detected.
But the team also found that pregnancy tests can also give incorrect results to women five or more weeks into their pregnancies when hormone levels tend to be very high.
Professor Ann Gronowski, medical director at Barnes-Jewish Hospital in Missouri, published the first paper highlighting the problem in 2009.
Along with colleagues, she published a paper evaluating how likely several pregnancy devices were to give false negative results.
Why do they happen?
Dr Gronowski explained that pregnancy tests work by detecting a hormone called human chorionic gonadotropin (hCG) in a urine sample.
But sometimes a “degraded form” of the hormone can also be picked up – leading to the false negative result.
She told the School of Medicine: “One antibody captures the hormone in the urine, and the other serves as a signal.
“It has a colour 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.
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
Dr Ann Gronowski
“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 colour when that happens, and therefore you get a negative result even though the hormone might be present.”
What should you do?
She advised against attempting to dilute the urine with water to get a more accurate result – something she says she came across in online pregnancy forums through her research.
“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.”
Dr Gronowski’s findings led the Food and Drug Administration in the States to change its standards for evaluating new pregnancy tests.
However, it only applies to new tests so the older devices with the false-negative problem are still on the market.
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