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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.

However, when we waited a full 10 minutes before reading the results, seven of the test kits performed much better than they did at the manufacturer's suggested waiting time. We've indicated those products in the Ratings (in the column headed "10-minute hCG sensitivity"). Given the extra time, five performed as sensitively as First Response and Clear Choice.

All the kits were easy to use. Three of them, First Response, Answer Quick & Simple, and ClearBlue Easy, produced result lines that were more intense than the others at lower hCG concentrations, making them the easiest to read.

EARLIEST DETECTION PROBLEMATIC
As several studies have made clear, these pregnancy-test kits may not work the same for all women. Even if the test kit is working perfectly to detect the hormone released after the embryo is implanted, many women will obtain incorrect results if they test immediately after missing their period.

POSITIVE OR NOT?
The darkness of the result line on pregnancy tests varies according to hCG level. Both the left and middle First Response tests are positive; the one at the left resulted from urine with a higher hCG concentration. The test on the right is negative. In all cases, the upper line indicates that the test has worked.

Photo by Don Stevenson

The main reason is that in 10 percent of pregnant women, the embryo does not implant until after the first day of the missed period, according to Dr. Wilcox's findings. Using urine tests to measure hCG concentrations in 136 normal early pregnancies during a study of possible environmental causes of early pregnancy loss, Wilcox found that even as late as a week after the missed period, 3 percent of pregnancies still hadn't implanted.

"Until implantation, it doesn't matter how sensitive the test is," Wilcox says. "You can't detect the pregnancy before it's producing the stuff that you're measuring, which is hCG."

Even pregnancies that have implanted may produce too little hCG for many at-home test kits to detect the day after the missed period when read after the waiting period specified in test package instructions. Some kits improve in detection when read after a wait of 10 minutes, but waiting longer than that may produce a negative result that looks faintly, misleadingly positive.

Finally, there's the issue of early pregnancy loss. "About a quarter to a third of pregnancies implant just for a short time and then fail," says Dr. Cole of the University of New Mexico. "That leads to a transient rise in hCG, which can extend for as long as two days after the day of the missed menstrual period."

All this means that a pregnancy test conducted as early as the day a period is overdue can be misleading, whether it's positive or negative. A test taken a week later will almost certainly be accurate.

SO WHAT'S WITH THE 99 PERCENT?

A SUPERIOR CHOICE: The First Response Early Result Pregnancy Test was both sensitive and reliable.

If at least 10 percent of pregnant women, and possibly even more, will come up with a negative pregnancy-test result the first day after their missed period, how is it that the kits can boast of 99 percent accuracy?

The answer involves a Food and Drug Administration regulation that allows makers of medical tests and devices, such as home pregnancy-test kits, to compare new products against older ones already on the market. To claim that a pregnancy test is 99 percent accurate, all a company has to show is that its results match up with the results of an existing test 99 percent of the time. And since technology has made the tests more sensitive over the years, it's easy for new products to match the accuracy of older, less sensitive products. Of all the test kits we evaluated, only the First Response package insert contained a clear explanation of its claims of accuracy: "In studies with urine samples representative of both pregnant and non-pregnant subjects, laboratory technicians obtained the correct expected result in more than 99% of the samples, and consumers obtained the correct expected result in 98.4% of the samples."

RECOMMENDATIONS
Women need to use home pregnancy-test kits with a clear understanding of their limitations. If you're comfortable waiting, a sensitive test taken a week after your period is overdue will almost certainly give you accurate results. If you elect to take the test as early as the day after you've missed your period, remember that a negative result isn't 100 percent certain. And a positive result may mean either a viable pregnancy or a pregnancy destined to end shortly after it began. With either of those results, you should plan on testing again a week later, just to be sure. For that reason, we recommend buying the tests in pairs; 15 of the 18 products we tested come in multiple-test kits.

As for your choice of product, we recommend the top-rated First Response test kit, which was both ultrasensitive and easy to read, though, at $18.09 per two-pack, it is also among the costliest of all the test kits. Or choose one of the brands that display at least very good sensitivity when read after a 10-minute waiting period, and carefully time a 10-minute wait before reading results. Don't wait any longer, however; after the 10-minute mark, a negative result may look faintly, misleadingly positive.

MANUFACTURERS' CLAIMS VS. REALITY
Our testing and analysis of the clinical evidence throws doubt on some of the claims manufacturers print on pregnancy- and ovulation-test-kit packages. While the following claims may be true for some women, they do not apply to all.

From: Consumers Reports 2003