A false positive pregnancy test occurs when a home or clinical test indicates pregnancy, but no viable pregnancy exists. This result happens because the test mistakenly detects the presence of human chorionic gonadotropin (hCG), the primary pregnancy hormone. Home pregnancy tests are designed to identify hCG in urine, which is produced by the cells that eventually form the placenta. While these tests are generally accurate, certain medications, medical conditions, and physiological factors can interfere with the chemical reaction, leading to a misleading positive reading.
Hormonal Medications That Mimic Pregnancy
The most direct and common drug-related cause of a false positive is the administration of synthetic hCG itself, typically through fertility treatments. These medications are given to stimulate ovulation or support the lining of the uterus after conception. Since the test is designed to find this specific hormone, the injected drug triggers a positive result regardless of actual pregnancy status.
This class of fertility drugs includes brand names like Ovidrel, Pregnyl, Profasi, and Novarel. These products contain chorionic gonadotropin, often called a “trigger shot,” which prompts the release of an egg from the ovary. The hCG from the injection enters the bloodstream and urine, where it is easily picked up by the home test.
The timing of testing is crucial following these injections. The synthetic hCG needs time to be metabolized and cleared from the body, a process known as the “washout period.” Testing too soon, generally within seven to fourteen days of the last injection, can result in a false positive. Healthcare providers recommend waiting a specific period to ensure the result reflects the body’s own hormone production, not residual medication.
Other Medications That Can Influence Results
While the direct introduction of hCG is the clearest mechanism for a false positive, certain other prescription drug classes have been implicated, although their effect is often indirect or less common. Some antipsychotic medications, such as chlorpromazine, perphenazine, and thioridazine, have been linked to false positive results. These drugs may interfere with hormonal balance or potentially affect the test’s antibody-based detection mechanism.
Anticonvulsants, specifically carbamazepine, are another drug class cited in rare cases of false positives. Additionally, high-dose diuretics and tranquilizers, including certain benzodiazepines, have been reported to cause interference. These non-hCG medications often cause a false positive by altering the concentration or chemical composition of the urine, or by cross-reacting with the antibodies used in the test strip.
The evidence for interference from these non-hCG drugs is generally less robust than for fertility treatments. Contamination of the urine sample with protein or white blood cells, such as from a urinary tract infection, can also interfere with the test chemicals and produce a misleading result.
Medical Conditions That Raise hCG Levels
Beyond medications, several physiological conditions unrelated to a viable pregnancy can cause the body to naturally produce hCG or hCG-like substances, leading to a positive test result. One significant category is Gestational Trophoblastic Disease (GTD), which includes molar pregnancies. A molar pregnancy is a non-viable mass that grows in the uterus due to abnormal fertilization. The abnormal tissue produces high levels of hCG, exactly like a normal placenta.
Certain rare cancers can also secrete hCG or its subunits, leading to a positive test. These malignancies are often of germ cell origin, such as choriocarcinoma, or occasionally involve the breast, ovary, kidney, or liver. The presence of these tumors causes the production of the hormone as a paraneoplastic syndrome, mimicking the hormonal signature of pregnancy.
Another factor is the presence of “phantom hCG,” which is caused by heterophilic antibodies in the person’s bloodstream. These antibodies interfere with the laboratory assay, binding to the test components and causing a false reading of elevated hCG. Finally, a biochemical pregnancy, which is a very early miscarriage, involves the brief production of hCG after implantation before the pregnancy fails, resulting in a temporary positive test.
Steps Following a False Positive Result
Receiving an unexpected positive result should prompt immediate follow-up with a healthcare provider for definitive confirmation. The most reliable method to confirm or rule out a false positive is a quantitative blood test, which measures the precise level of hCG in the blood. Unlike a home urine test, a blood test can also track whether the hCG level is rising or falling, providing insight into the underlying cause.
Healthcare providers may also recommend retesting after a specific interval, especially if a medication is suspected to be the cause. If the result is positive but the person is not pregnant, a medical workup is necessary to investigate potential underlying conditions that abnormally elevate hCG levels.
Practical considerations should also be reviewed, as user error is a common factor in inaccurate results. This includes checking the expiration date of the home test or ensuring the result was read within the recommended time frame.