Baby Hopes explains that an invalid positive pregnancy test can occur because of a chemical pregnancy, an evaporation line on the test, chemical interference or a missed reaction time. Baby Hopes says that approximately one out of four pregnancy tests that claim to be positive end in a chemical pregnancy.
According to Baby Hopes a chemical pregnancy is an early miscarriage that occurs shortly after implantation. Due to early pregnancy tests, a woman can see a positive test result although the pregnancy does not last.
Missed reaction time, according to Baby Hopes, means that a woman read the test result after the recommended time. Tests can become invalid after a specific time stated in the instructions because additional lines sometimes show up. These extra lines do not indicate a positive test.
Those who use hCG, also known as Human Chorionic Gonadotropin fertility treatment, must wait until the shot has had enough time to metabolize. If a woman takes a pregnancy test before the recommended waiting period, the test is considered invalid: It can give a false positive result, according to Baby Hopes.
As urine passes over the test strip, sometimes an evaporation line appears. This extra line is often misinterpreted as a positive result. Baby Hopes explains that an easy way to determine if this line is an evaporation line is to see if the line is a different color than the test instructions state.
WebMD states that sometimes a false negative result occurs when a woman takes a test too soon before her missed menstrual period. A false negative may result if the test is expired, the woman took the test incorrectly, the woman's medications interfere with the test or the urine sample is too diluted.