Home pregnancy tests detect a specific hormone in the urine to check for pregnancy. While pregnancy is the most common cause of a positive result, certain medical conditions, including rare types of cancer, can produce the same detectable hormone. An unexpected positive test warrants medical follow-up to determine the source of the hormone and rule out underlying health issues.
Understanding Human Chorionic Gonadotropin
Home pregnancy tests detect Human Chorionic Gonadotropin (hCG). This hormone is primarily produced by the placenta after a fertilized egg implants, supporting the early embryo. HCG levels increase rapidly in the first trimester, often doubling every 48 to 72 hours. HCG is a glycoprotein composed of alpha and beta subunits. Since the alpha subunit is shared with other hormones, home tests specifically target the unique beta subunit to confirm hCG’s presence.
Malignancies That Produce hCG
Certain types of cancer can produce hCG, leading to a positive result even when pregnancy is not present. The most common malignancies associated with this production are Germ Cell Tumors, which can originate in the testes or ovaries. These tumors often produce the hCG hormone because they are derived from cells that have the potential to differentiate into various cell types, including the trophoblast cells that form the placenta.
Gestational Trophoblastic Disease (GTD), a group of pregnancy-related tumors including choriocarcinoma and molar pregnancies, is known for producing high levels of hCG. Choriocarcinoma is a potent hCG producer, sometimes reaching very high concentrations. Non-seminomatous testicular cancers and ovarian germ cell tumors also produce elevated hCG, where the hormone serves as an important tumor marker for diagnosis and monitoring.
Less commonly, non-gonadal cancers can also produce the hormone, typically in lower amounts. These extragonadal malignancies may include cancers of the bladder, kidney, lung, liver, breast, and gastrointestinal tract.
Common Non-Cancer Causes of a Positive Result
While cancer is a possibility, statistically, other non-life-threatening factors are far more likely to cause an unexpected positive result. A common issue is the presence of an evaporation line, which is a faint, colorless line that can appear on a test strip after the urine has evaporated. This line is often mistaken for a true positive, especially if the test result is read after the recommended time window.
Certain medications can also interfere with the test by containing the hormone itself. Fertility treatments, such as those used to trigger ovulation, often involve an injection of synthetic hCG. Testing too soon after this injection can result in a positive reading that reflects the medication in the system rather than a true pregnancy.
In women approaching menopause, the pituitary gland may produce low levels of hCG while compensating for declining ovarian hormone production. Another rare cause is heterophilic antibodies, sometimes called “phantom hCG.” These antibodies can bind to the test components, causing a false signal that mimics the hormone.
Medical Steps Following an Unexpected Positive
Any unexpected positive pregnancy test requires medical evaluation to determine the hormone source. The first step involves a quantitative blood test (beta-hCG test), which measures the precise amount of the hormone in the bloodstream. Unlike qualitative home urine tests, this blood test provides a numerical value essential for monitoring.
The physician may repeat the quantitative test after 48 to 72 hours to check if hCG levels are rising as expected for a viable pregnancy. Low or non-increasing levels may indicate an early pregnancy loss, an ectopic pregnancy, or a non-gestational cause. An ultrasound is often performed to confirm the pregnancy location or rule out ectopic implantation.
If pregnancy is ruled out, a differential diagnosis investigates other possible causes, including medication side effects or an underlying tumor. Further specialized testing, such as tumor marker panels and imaging studies, may be ordered to identify the source of the aberrant hormone production.