Home pregnancy tests (HPTs) are designed to detect a specific biological marker, leading to frequent concerns about whether a sexually transmitted disease (STD) can cause a false positive result. Understanding how these tests work helps demystify the process and provides a scientific basis for the results. The immediate concern about a false positive can be addressed by examining the distinct biological processes of infection and gestation.
Why STDs Do Not Cause False Positives
Sexually transmitted diseases (STDs) do not produce the hormone required to trigger a positive pregnancy test result. These infections are caused by foreign pathogens, including bacteria, viruses, or parasites, such as Chlamydia trachomatis or HPV. These organisms are chemically and structurally distinct from human hormones.
The processes of infection and human reproduction are fundamentally separate at a molecular level. A pathogen’s presence in the body, even if it causes inflammation, does not involve the production of pregnancy-specific hormones. Therefore, the body’s response to an infection cannot mimic the chemical signature of a developing pregnancy.
How Home Pregnancy Tests Detect Pregnancy
Home pregnancy tests function by detecting Human Chorionic Gonadotropin (hCG), often called the pregnancy hormone. This hormone is produced by the cells that eventually form the placenta, beginning shortly after a fertilized egg implants in the uterine wall. The HPT is a lateral flow immunoassay specifically calibrated to recognize this molecule in the urine.
The test strip contains monoclonal antibodies chemically bonded to a dye. When urine is applied, hCG binds to these mobile antibodies, forming a complex that travels up the strip. This complex reaches a fixed test line containing a second set of antibodies that capture it. This “sandwich” causes the colored dye to concentrate, producing the visible positive result. The test is highly specific, meaning only the hCG molecule can complete this binding process and activate the result line.
Medical and Chemical Causes of False Positive Results
Since STDs do not produce hCG, a positive test result when pregnancy is not viable is linked to other medical or chemical interferences. One common non-pregnancy reason is a chemical pregnancy, which is a very early miscarriage. This occurs when a fertilized egg briefly implants, producing enough hCG to register on a test, but the pregnancy fails shortly thereafter.
Fertility Treatments and Medications
Another frequent cause is the introduction of artificial hCG into the body, often through fertility treatments. Patients receiving an hCG trigger shot to stimulate ovulation may have the injected hormone linger in their system for up to two weeks. Testing before the injected hCG has fully cleared can lead to a false positive reading. Certain medications can also interfere with the test’s chemistry or affect hormone levels, though this is less common.
Rare Medical Conditions
More rarely, a positive result may be caused by a medical condition that produces hCG. Examples include gestational trophoblastic disease (a non-viable uterine growth) or certain types of tumors, such as germ cell tumors. These cells can ectopically secrete the hormone, tricking the test into reading a positive.
User Error
Finally, user error is a factor, particularly the misinterpretation of an evaporation line. This is a faint, colorless streak that can appear after the test’s designated reading window when the urine dries.
Interpreting Results and Next Steps
An unexpected positive result warrants immediate attention to confirm the finding and determine the true cause. The most reliable first step is to repeat the test with a new HPT, preferably using the first morning urine, which contains the highest concentration of hormones. Following the test instructions precisely, especially regarding the reading time, minimizes the risk of misinterpreting an evaporation line.
If the positive result persists, the next step is a visit to a healthcare provider for a definitive blood test. A quantitative hCG blood test measures the exact amount of the hormone and tracks whether levels are rising, indicating a viable pregnancy, or falling, suggesting a chemical pregnancy or recent loss. Consulting a medical professional is also important to rule out rare medical conditions or complications like an ectopic pregnancy.