New home testing devices, like the rapid COVID-19 test, often spark questions about alternate uses. A rumor circulating online asks whether a COVID-19 test can function as a pregnancy test. This idea is understandable since both are easy-to-use lateral flow devices that display a line for a positive result. However, a COVID-19 test cannot accurately detect pregnancy. This impossibility is rooted in the precise biological mechanisms that make each test uniquely suited to detect its intended target molecule.
The Functioning of Pregnancy Tests
Home pregnancy tests rely on detecting Human Chorionic Gonadotropin (hCG), a specific hormone. This hormone is produced by the developing placenta immediately after the fertilized egg implants. hCG concentration increases rapidly in the body and is excreted into the urine where the test can detect it.
The test is an immunometric assay, utilizing antibodies engineered to bind specifically to the hCG molecule. If hCG is present in the urine sample, it binds to mobile antibodies conjugated with a colored particle. This complex travels up the test strip by capillary action, where stationary antibodies capture it, forming the visible positive line.
The Functioning of COVID-19 Antigen Tests
The rapid COVID-19 antigen test detects proteins that are part of the SARS-CoV-2 virus structure. These tests predominantly target the viral nucleocapsid (N) protein, identifying a current, active infection by detecting these viral components in a sample, typically collected via a nasal swab.
The COVID-19 test is also a lateral flow immunoassay, but its internal components are different. The test strip is coated with specialized antibodies created to bind only to the SARS-CoV-2 viral antigens. If viral proteins are present, they form a complex that migrates to the test line. Fixed antibodies capture this complex, resulting in a colored line that indicates a positive result.
Why Molecular Specificity Prevents Interchangeability
The fundamental reason these tests cannot be swapped is molecular specificity, often described as a “lock-and-key” mechanism. Antibodies are the “keys” manufactured to fit only one specific “lock.” Pregnancy test antibodies recognize the unique structure of the hCG hormone, while COVID-19 test antibodies bind distinct, non-human viral proteins.
The molecular structure of hCG is vastly different from that of a SARS-CoV-2 protein. This difference makes it impossible for the viral-protein-specific antibodies to bind to the pregnancy hormone, as the two target molecules are structurally unrelated.
Any line appearing on a COVID-19 test exposed to urine is due to a non-specific chemical reaction, not biological detection of hCG. Factors like the urine’s acidity (pH) or the presence of salts can degrade the test strip’s chemical reagents, causing a false color change. People should only use tests approved for their intended purpose.