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April 29, 2004
News this month Most home pregnancy tests claim to be 99 percent accurate and imply that you can use them successfully as early as the first day of a missed menstrual period, but new research indicates these claims are misleading. In fact, a study published in the American Journal of Obstetrics and Gynecology (ACOG) found that only one of the 18 brands of home pregnancy tests (HPTs) tested was sensitive enough to detect pregnancy at this time.
Only one of the 18 brands of home pregnancy tests (HPTs) tested was sensitive enough to detect pregnancy on the first day of a missed menstrual period.
HPTs detect the presence of human chorionic gonadotropin (hCG) in a woman's urine. HCG is a hormone produced by the placenta during pregnancy. The amount of hCG increases rapidly over the first weeks of pregnancy, doubling every two days. Pregnancy tests that detect the lowest number of units of hCG in the urine are the most sensitive and produce the earliest confirmation of pregnancy. Investigators from the University of New Mexico first calculated the hCG levels found in 25 women in early pregnancy. They collected urine samples of these women between four weeks and four weeks and three days after their last period. The investigators found a broad range of the pregnancy hormone in this group, ranging between 23 and 2,348 milli-International Units per milliliter (mIU/mL). The median level was 49 mIU/mL, and the fifth and 95th percentiles for hCG were 12.4 and 241 mIU/mL, respectively. They determined that in order for a test to be 95 percent accurate in detecting hCG during the first day of a missed period, the test had to be sensitive to the presence of 12.4 mIU/mL of hCG in the urine. Dr. Laurence Cole and his colleagues evaluated 18 currently available tests. They were purchased at different retail chains to minimize lot-to-lot variations. All of the tests used similar mechanisms to detect pregnancy and had both a control window and a window to demonstrate positive results.
Most of the tests were accurate a week after an expected period was due.
Researchers found only one brand was consistently sensitive enough to detect 12.4 mIU/mL of hCG, the level they determined most pregnant women were likely to have on the first day of a missed menstrual period. Two of the brands gave false-positive or invalid readings if faintly discernible results were considered. Only 44 percent of the brands gave clear positive results at 100 mIU/mL hCG; 83 percent of brands were positive at that level if faintly discernible results were included. All of the brands detected pregnancy at that level if both faintly discernible results and extended reading times were included. Most of the tests were accurate a week after an expected period was due. “Since their introduction in 1975, the number of commercially available HPTs has increased from four to more than 20 brands,” wrote Cole. “The objective of this investigation was to determine whether there is validity in the claim advertised by all HPT brands of >99 percent accuracy and use of the test at the first day of the missed period.” According to current Food and Drug Administration (FDA) regulations, in order to claim that a HPT is greater than 99 percent accurate, the manufacturer need only demonstrate that the test performs as well in the lab as previous products. Since today's HPTs are more sensitive than previous products, it's not surprising they are able to maintain this claim, but it has nothing to do with a test's ability to detect pregnancy on the first day of a missed period.
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For the 12th year in a row, Yale-New Haven has been highly ranked by U.S. News & World Report for its programs in gynecology. |
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 Limitations of home pregnancy tests I tell my patients not to even call me about a possible pregnancy until they are at least a week late because we know home pregnancy tests (HPTs) are not likely to be reliable until that time. If it is important for a woman to know if she's pregnant earlier than this, I would advise her to have a blood test. Blood tests are 100 percent accurate at detecting pregnancy even at this very early point. Interpretation issues In addition, some of the brands are not very user friendly and are subject to reader interpretation. Most entail reading a test strip that has been exposed to urine by detecting a pink or blue line or a red plus or minus sign and comparing it to a sample line. Did I see pink line, a red line, a faint line?
“The best time to test is first thing in the morning when the level of hCG is most concentrated in the urine.”
It's been my experience that a woman taking an HPT on the first day of a missed period is likely to be anxious about being pregnant. There is enough ambiguity in the result-reading process for that anxiety to affect how she interprets the results. It's also important to wait at least the amount of time noted in the directions before checking results, but some brands indicate an evaporation line may develop if the test is allowed to sit too long, which may make results difficult to interpret. Unpredictable menstrual cycles may also make it difficult to determine when to test for pregnancy. If a woman has a consistent 28-day cycle, home testing is likely to be reliable on day 35 of her cycle. If a woman's periods vary between 24 and 35 days, an HPT should be done on day 42. In our experience, the easiest tests for a woman to use at home are those that entail placing a test strip in a stream of urine. In the office we collect a sample of urine from a woman and then put a few drops in a collection cup for testing with a gradient strip. The best time to test is first thing in the morning when the level of hCG is most concentrated in the urine. Qualitative and quantitative blood tests for hCG Technically, women will start producing some level of hCG at the time of fertilization, but the level begins to rise significantly when the embryo is implanted in the uterus, which is generally four to five days after fertilization or around day 18 for women who have a 28-day cycle. When we talk about the first day of a missed period, that's about 10 days after an egg implants in the uterus. The most sensitive HPTs may detect pregnancy at that time; blood tests may be accurate even before a period is late. Slow-rising hCG can be related to an impending miscarriage or ectopic pregnancy, so we generally monitor the hCG levels of pregnant women who have had pregnancy losses or who experience spotting. Levels of hCG double about every two days up to 12 weeks of pregnancy. If the levels are not climbing, we investigate further to determine if a pregnancy loss has occurred. Chemical pregnancies Practical tips
Dr. Ravski is an attending gynecologist/obstetrician at Yale-New Haven Hospital and an assistant clinical professor in obstetrics and gynecology at the Yale School of Medicine. He is a partner of the County Obstetrics and Gynecology Group with offices in New Haven, Branford, Clinton and Wallingford.
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