Q: What are the reasons for a false positive or false negative pregnancy test result?
A: A pregnancy hcg test checks for the presence of the pregnancy hormone hCG or human chorionic gonadotropin in your body. A false positive hCG pregnancy test, a positive hCG test when you are not pregnant, rarely happens, but if it happens then it can be seen both with blood and urine tests, though in each case, the causes can be slightly different.
- A false positive pregnancy test means that your pregnancy test is positive but you are NOT pregnant.
- A false negative pregnancy test means that you test negative but you ARE pregnant.
Most women who have a positive pregnancy test are pregnant and only a few are not. One cause of a false positive pregnancy test is that you do not read your urine pregnancy test correctly. Pregnancy tests have certain instructions and before you make the diagnosis of a positive pregnancy test make sure that you followed all instructions. Most manufacturers suggest to read their test after 3 minutes and to NOT READ it after 10 minutes. Once you have a negative test after 3-5 minutes THROW OUT the test. Any positive test appearing only well after that time is inaccurate and cannot be considered positive.
False positive urine tests can also occur under the following circumstances:
- Fertility treatment: If you receive shots of hCG or gonadotropins (for example, Pregynl, Profasi, Pergonal, APL) for ovulation it's posible to have a positive urine or blood pregnancy test for two to three weeks after the shot and not be pregnant. A rising blood hCG level usually indicates a pregnancy.
- Evaporation line. If you read the test after the suggested time has passed.
- Improper Testing - When taking a home pregnancy test, it is very important to read the directions before testing. Your home pregnancy test instructions will specify when test results should be interpreted and when test results are no longer valid. Some tests will produce a faint positive test result if read after the instructed time. You should use a clock or a watch and record the time you perform the test and read results at the instructed time. Guessing how long it has been since performing the test can lead to false results. Three minutes can seem like twenty minutes and vice versa when you are anxiously waiting to find out if you are expecting.
- Chemical pregnancy - This is a pregnancy that has a positive pregnancy test but the embryo or fetus do not develop
- Certain rare medical conditions such as trophoblastic disease, islet-cell tumors
- Medications - Some drugs like Methadone, chlordiazepoxide, or promethazine can give a false positive pregnancy test
- "Phantom-hCG" The most important though rare false positive blood test is a where a woman has a low positive blood pregnancy test but she is not pregnant and the urine test is negative.
- Defective Pregnancy Test - Expired tests or faulty home pregnancy tests can also lead to false positive results. Always check the expiration date before testing to assure accurate results. Keep in mind, even with the highest quality control, an occasional defective test may occur.
If you have tested positive and are unsure about the results of your home pregnancy test, consult your doctor.