In general, when used as the instructions indicate, home pregnancy tests are as much as 90% or more accurate at demonstrating pregnancy. Most brands run at around 97% accuracy when used correctly. However, there are some times when you might get a false reading on your home pregnancy test.
First, it is important to understand how exactly home pregnancy tests work. A home pregnancy test measures the hormone human chorionic gonadotropin, also known as hCG. For a pregnancy test to show positive, your body must be producing a level of hCG that the test can detect. Not all pregnancy tests are the same, however. Some tests are more sensitive than others and will show a positive result earlier than tests that are less sensitive. The box that the test came in should indicate somewhere what its sensitivity is. Sensitivity is measured in IU/L, which stands for milliInternational Units per Liter. A very sensitive test may have a sensitivity of 20 IU/L, whereas a test that is not as sensitive might have a sensitivity of 50 IU/L.
Because of the sensitivity issue, the timing of when you take a home pregnancy test can determine its accuracy. A very sensitive test may be able to produce a positive results as little as 7 days after ovulation. Implantation must occur before hCG is produced, and implantation will generally take place 6-12 days after ovulation. Thus, it makes sense to try to wait at least 12 days after ovulation to test for a reliable result. If you cannot wait that long and have other symptoms of pregnancy, you might use a test that is higher in sensitivity, but keep in mind that even if the result is negative you may still be pregnant, but just not implanted.
There are other things that can cause a home pregnancy test to indicate pregnancy when you are not actually pregnant. In some cases, you might have a chemical pregnancy. You might be experiencing evaporation lines, or drugs that you use for fertility may interfere with your test.
A chemical pregnancy is, essentially, a very early miscarriage. Some studies have suggested that as many as half of all first pregnancies end in miscarriage, and this often happens in the first few weeks of pregnancy. In this situation, without a home pregnancy test, the mom-to-be would never have known she was pregnant to begin with.
Evaporation lines are created on a pregnancy test as the urine stream crosses the test. The urine on the test sometimes evaporates, and leaves a mark on the test results area. This typically shows up as a faint, grey or even colorless line in the test area. It is easy to mistake this line as a positive result when you may not actually be pregnant. No particular brand of home pregnancy test is immune to evaporation lines, although some home pregnancy tests may be better than others at avoiding these.
Finally, the use of some medications, especially fertility medications, have the pregnancy hormone hCG (Human Chorionic Gonadotropin). This is the hormone measured by a home pregnancy test, and such medications could affect the accuracy of a home pregnancy test.