Whether a woman is trying to conceive, trying desperately not to conceive, or merely curious as to why she's missed a period, at some point she's likely to turn to a home pregnancy test for answers. First Response pregnancy tests are popular because give they women options---Early Response tests claim to be able to detect pregnancy up to six days prior to a missed period, while Rapid Results tests deliver the information in under a minute. Of course, as important as it is to get information early and quickly, the most important feature of a pregnancy test is that it be accurate.
First Response tests for the presence of the hormone human chorionic gonadotropin, or hCG. This hormone is not produced by adults. It is produced only by the chorion---part of the placenta---of a developing embryo. As such, it must be present in the urine of a pregnant woman, and if it's present in a woman's urine, the only explanation (unless she's taking a fertility drug that mimics hCG) is that she's pregnant. From a theoretical standpoint, then, the accuracy of the First Response test should be quite high, and the rate of false positive or false negative should be quite low.
The First Response website provides a host of information regarding the test itself, including the claim that the accuracy of the tests is over 95% for the Rapid Results version, and approaching 100% for the Early Results version. The site does report some caveats, however. A women's hormone levels vary, and not everyone has enough hCG in their urine at a given point in early pregnancy for the test to report a positive result. Further, the site notes that the Early Results version of the test is only as much as 40% accurate six days prior to a missed period, with test accuracy increasing significantly as the date of the expected period approaches.
Further complicating the accuracy of the First Response test is that false positives sometimes result from a phenomenon called an "evaporation line." This line, which mimics a positive result, can form when a test develops longer than the manufacturer recommends, according to Pee On A Stick, an information website for home pregnancy tests. Pee On A Stick notes that it's important to follow instructions closely when using home pregnancy tests in order to ensure accuracy, and First Response tests are no exception.
As heartbreaking as a negative pregnancy test can be to a woman trying to conceive (and as comforting as it can be to a woman trying not to), it's important to remember that the First Response test depends upon sufficient hCG in the urine to function accurately. Some women clear hCG from the blood into the urine slowly, and others not at all. For this reason, there are women for whom the First Response test won't show positive until well into pregnancy, and others for whom it will never display a positive result.
While First Response claims that its tests are 95 to 100% accurate on the first day of a missed period, this is predicated on the notion that a menstrual cycle should be 28 days, with ovulation occurring on day 14 (and implantation of a fertilized egg, if there is one, occurring 7 to 10 days after ovulation). The fact is, however, that women vary tremendously in their individual cycle physiology. It's entirely possible that by the first day of a woman's missed period, she might be pregnant but not yet have experienced implantation. A 2001 article in the "Journal of the American Medical Association" reports that the highest accuracy a pregnancy test can possibly have on the first day of a missed menstrual period is 90%, as 10% of embryos have not yet implanted by that time, and therefore can't begin to secrete hCG.