Finds One Chance in 10 of False Negative
A negative result on a home pregnancy test can be misleading if it’s done very early, a government study finds.
In a study of 221 women, 10 percent of the negative readings of tests on the first day of a missed period turned out to be wrong, says a report in the Oct. 10 issue of The Journal of the American Medical Association.
“If you use the test exactly as recommended on the package, there is a chance of a false negative. We’re not sure of the exact percentage, but it is at least 10 percent,” says study leader Dr. Allen J. Wilcox, senior investigator at the National Institute of Environmental Health Sciences.
The false negative results occur because the time for the early embryo to become implanted varies from woman to woman, Wilcox says. Home pregnancy tests all measure levels of human chorionic gonadotropin (hCG) in the urine. Levels of hCG increase substantially after the embryo is implanted, and the time of implantation can range from six to 12 days after ovulation, so the very first sign of pregnancy “is inherently difficult to predict,” the report says.
All the women in the study were trying to become pregnant. They kept track of their menstrual cycles and provided urine samples for later analysis. Of the 136 pregnancies, the first-day tests missed 14.
The researchers say they used a special test designed to be exquisitely sensitive to hCG levels. Considering that the best tests on the market are 100 times less sensitive than the one used in the study, a 10 percent error rate is as good as is possible, they say.
Tests on the market “differ in sensitivity and to some degree in the kind of molecule they test for. There are different varieties of hCG. There are literally dozens of kits on the market, and none of them include the level of sensitivity” of the laboratory test, says Wilcox.
Even the kit instructions vary widely. “Some package inserts say that if a test is negative, you are probably not pregnant. One says you are NOT PREGNANT’ (emphasis in the original). This unfounded assurance could have important consequences,” the researchers say.
A false negative might be dangerous for some women, Wilcox says. “If a woman has a job where she is exposed to things that might not be good for pregnancy — say a nurse who works with chemotherapeutic agents — a false negative might tell her not to worry,” he says.
The chance of a false result is highest for adolescents and young women because they have the greatest rate of delayed ovulation and implantation, the researchers say.
Delaying the test gives much better accuracy, but not perfection. Tests seven days after the first day of the missed period were 97 percent accurate, the study found.
Dr. Paul D. Blumenthal, associate professor of obstetrics and gynecology at Johns Hopkins School of Medicine and a member of the board of Planned Parenthood, says the study has a crucial message: “Listen to your body.”
“Those women who ovulate irregularly and have a false negative test because they don’t recognize their own ovulation may be falsely reassured by a negative test. If you feel pregnant a week later, don’t rely on that test,” says Blumenthal.
He says a second message is that a woman trying to achieve pregnancy is wise to behave as if she is already pregnant — avoiding alcohol, not smoking and taking other measures to avoid risk to an unborn child.
“What we preach now is that if you want to become pregnant, change your behavior accordingly. By the time you know you are pregnant, there may be several days of exposure for the embryo,” Blumenthal says.
What To Do
Wilcox says, “If you get a negative result on the first day of your missed period, don’t regard that as ironclad proof that you are not pregnant. If your period does not arrive a few days later, do the test again.”
In case you are interested in getting pregnant, find ways to boost opportunities of conceiving a baby by discovering facts about menstrual cycles, signs of pregnancy.