What is a false pregnancy test result?
A pregnancy test checks for the presence of the pregnancy hormone human chorionic gonadotropin (hCG) in your body, usually in the urine or blood. Both false negative and false positive tests are possible.
False-positive vs. false-negative pregnancy test
- A false positive pregnancy test means that your pregnancy test is positive but you are NOT pregnant.
- A false negative pregnancy test means that your pregnancy test is negative but you ARE pregnant.
- A true positive pregnancy test means that your pregnancy test is positive and you are pregnant.
- A true negative pregnancy test means that your pregnancy test is negative and you are NOT pregnant.
False-positive pregnancy tests are rare. Most women who have a positive pregnancy test are actually pregnant and only a few are not.
One frequent cause of a false positive pregnancy test is simply reading the home pregnancy test incorrectly. Pregnancy tests have certain instructions and they also differ from one another. Most manufacturers suggest you 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 well after that time is inaccurate and cannot be considered positive. However, a false positive pregnancy test can also happen after an hCG injection.
Besides errors doing and reading the urine pregnancy test incorrectly, there are only a very few circumstances in which you can have a positive pregnancy test (false positive) and you are not pregnant.
False-positive urine tests can also occur under the following circumstances:
Fertility treatment: If you receive shots of hCG or gonadotropins (for example, Pregnyl, Profasi, Pergonal, APL) for ovulation, it's possible 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. This is what happened to Kim Kardashian.
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 Your home pregnancy test instructions will specify when test results should be interpreted and when test results are no longer valid.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 does not develop.
Miscarriage or after pregnancy: Very early pregnancy loss or miscarriage. Also, after a pregnancy or miscarriage, the hCG pregnancy hormone can stay for weeks in your body.
Medications: Some drugs like Methadone, chlordiazepoxide, or promethazine can give a false positive pregnancy test.
What is a phantom hCG which causes a false positive pregnancy test?
Though very rare, a false positive blood test happens with a low positive blood pregnancy test, the urine test is negative, and there is no pregnancy.
A "Phantom hCG" causes a positive blood pregnancy test without being pregnant. Patients with a phantom hCG will have a persistently yet not significantly rising positive blood pregnancy test while the urine test is negative.
The hCG test, or pregnancy test, is one of the most common tests done in laboratories. It is assumed that an hCG assay detects only hCG or hCGß. This is not always the case. hCG kits can detect a wide and varying range of different hCG-related molecules in serum or urine samples.
- hCG dissociation and degradation products, like the free ß-subunit, nicked hCG and
- ß-subunit core fragment
- hyperglycosylated hCG a variant of hCG with separate biological functions and
- a pituitary gland-produced variant of hCG
The reason for a phantom hCG is that some individuals react falsely positive to some substrate in the blood hCG test and thus will display a consistently low positive blood pregnancy test even though they are not pregnant, but their urine test will be negative. This phantom hCG may lead to serious misdiagnosis and intervention but can be detected with serial dilutions. Patients with phantom hCG have a positive blood hCG but a negative urine hCG test. If there is ever a question, a urine test should be done together with the blood hCG test.
Some women age 50 and older in menopause may have a slightly elevated blood hCG test, which usually is not associated with pregnancy.
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.