I just read this really interesting bit of info on false negatives - it's giving me comfort and hopefully will for others:
Consumer Reports Reviews of Home Pregnancy Tests
We tested 18 pregnancy-test kits to find out which brands work best. Buy the top performers and follow our tips for the most accurate results.
The claims on home pregnancy-test kits certainly sound impressive: "99 percent accurate," "results as early as 1 minute," "ready to use the first day of missed period." But how trustworthy are they? Our tests found that for many women, home pregnancy-test kits will not work as accurately, as quickly, or as soon after the missed menstrual period as the labels promise.
We also found that the usefulness and accuracy of home pregnancy-test kits can be improved if women are aware of the kits' limitations and learn how to work around them. For example, though most test-kit labels advise women to test "as early as the first day of missed period," the kits are more accurate when used a week later.
For women who consider it important to test at the very first suspicion of pregnancy, our tests identified one especially sensitive and reliable brand, First Response Early Result Pregnancy Test. It will detect the very low levels of the hormone likely to be present in the earliest days of pregnancy.
However, First Response promises on its package to detect pregnancy several days before a missed period. At that point, 26 percent of viable pregnancies are not yet producing any pregnancy hormone whatsoever, according to a 2001 report by Allen J. Wilcox, M.D., Ph.D., of the National Institute of Environmental Health Sciences, a federal research laboratory in Durham, N.C.
TESTING THE TESTS
All home pregnancy-test kits use monoclonal antibodies to detect a hormone known as human chorionic gonadotropin (hCG), which is produced by the developing placenta beginning the day on which the embryo implants in the uterine wall. Concentrations of this pregnancy hormone vary widely, depending on the individual. A recent study of pregnant women, conducted by Laurence A. Cole, Ph.D., Chief of the Division of Women's Health Research at the University of New Mexico, found that urinary hCG ranged from 23 to 652 mIU/ml (thousandths of an International Unit per milliliter) 28 days after their last menstrual period. During the first several weeks of pregnancy, hCG concentrations in blood and urine increase exponentially, doubling every two to three days.
All 18 brands of pregnancy kits we tested employ similar technology. Most use sticks with absorbent wicks that are held directly in the urine stream. Most also allow the user to collect the urine in a cup and then dip the stick. One brand, the Inverness Medical Early Pregnancy Test, offers a choice of two products, one with a stick and one with a two-step process in which the user first collects urine in a cup, then uses a medicine dropper to transfer a few drops to a plastic cassette.
Regardless of collection method, reading the results is the same for every test kit: If a line, however faint, appears in the result window after a specified number of minutes, the test is positive. (The kits also have a second, "control" line that appears when the test is working properly.)
To assess the sensitivity of the test kits, we worked with an independent laboratory that specializes in hCG studies. We spiked hCG-free urine with varying concentrations of the hormone, to mimic the range found in normal early pregnancies. We tested each product at increasing hCG concentrations until we obtained a positive result.
We conducted the tests as instructed on each product's package insert. Technicians read the results after the specified minimum wait--anywhere from 1 to 5 minutes, depending on the product--and then again at the maximum time allowed (10 minutes in all the products we tested). They also evaluated each kit on how easy it was to read a positive result, and on overall ease of use.
HOW THE TEST KITS PERFORMED
One product, First Response Early Result Pregnancy Test, emerged as the most reliable and sensitive test kit. It detected hCG at concentrations as low as 6.5 mIU/ml. That's almost certainly sensitive enough to detect any pregnancy soon after implantation. One other product, Clear Choice At Home Pregnancy Test, which provides a small jar for urine collection, was able to detect the same low hCG levels at the reading time specified by the manufacturer. But several of the Clear Choice kits we tested failed to work (showed no line in control window), making it less reliable than the First Response product.
The other test kits were less sensitive than those two; the five least sensitive couldn't detect hCG below concentrations of about 100 mIU/ml at their specified reading times.