Home pregnancy tests and blood pregnancy tests taken at a doctor's office measure the presence of the hCG, a hormone secreted by the developing placenta shortly after fertilization. According to WomensHealth.gov, home pregnancy tests are highly accurate when used correctly and blood tests are even more so. There are a handful of factors, though, that can lead to pregnancy test errors. An understanding of these can help you understand what not to do when administering a test.
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Failure to Follow Instructions
Whether you purchase a kit that requires you to dip a stick in urine or one that requires a steady urine stream to saturate it, precision is key. If liquid inadvertently comes in contact with the area where results are read, it can cause an error. Avoid this mistake by holding the test window away from your urine stream, or by dipping your stick only to the point indicated on the directions. Be also mindful to read the results of a home pregnancy within the alloted reaction time—usually a few minutes. Tests read after the reaction time, or tests used after the expiration date, can be incorrect. Read directions thoroughly prior to beginning a test, and follow the directions to the T until results have been read.
Urine that has been diluted by liquids can give a false negative when using home pregnancy results. Avoid this error by administering your test first thing in the morning prior to urinating, and remember not to drink lots of water before sleeping or during the night. If you can't wait until morning, at least hold your urine for an hour or two before testing.
Other Reasons for False Readings
Medications used to treat infertility or perphenazine used to treat severe emotional and mental disorders, can give false results. In addition, certain forms of uterine, breast and ovarian cancer can elevate hCG levels and lead to a false positive test.