At-home pregnancy tests detect the presence of the hormone human chorionic gonadotropin (hCG) in urine. These devices use specialized chemical strips to provide a rapid result, typically within minutes. Tests have a defined shelf life determined by the stability of their internal components. Using a test past its printed expiration date significantly compromises its reliability, increasing the likelihood of an inaccurate result.
The Science Behind Expiration
The core mechanism of a pregnancy test relies on immobilized antibodies, which are biological proteins engineered to specifically bind to the hCG hormone. These antibodies are applied to a nitrocellulose membrane, forming the colored test line and the control line. Since antibodies are proteins, they are susceptible to denaturation, a process where the protein structure breaks down and loses its specific binding function over time.
This structural degradation prevents the antibodies from effectively recognizing and capturing hCG molecules in the urine sample. Manufacturers determine the expiration date based on stability testing, establishing the period during which the test is expected to perform accurately. The rate of antibody denaturation is accelerated by environmental factors such as high temperatures, direct sunlight, and excessive humidity.
Improper storage conditions can cause degradation even before the printed expiration date. Factors like heat and moisture can cause the antibody components to aggregate or fragment, reducing their potency. Once the antibody proteins degrade, the test’s ability to function as designed is reduced, decreasing its sensitivity to low levels of hCG.
Understanding Potential Outcomes
The most frequent outcome of using an expired pregnancy test is a false negative result, meaning the test indicates non-pregnancy even when hCG is present. This occurs because degraded antibodies are unable to capture enough hCG molecules to produce a visible reaction on the test line. This reduced sensitivity means the test may fail early in pregnancy when hCG levels are below the detection threshold of the compromised device.
A fresh test might accurately detect hCG concentrations as low as 25 mIU/mL, but an expired test may require a significantly higher concentration to show a positive result. This failure to detect lower hormone levels is the primary risk associated with using an expired test, potentially delaying necessary medical care. The materials of the strip itself, such as the paper or digital components, can also degrade, leading to faulty readings or error messages.
A less common outcome is a false positive result. This is often due to the formation of an evaporation line, which appears after the designated reading window. Evaporation lines form when the solvent dissipates, leaving behind a faint, colorless residue that mimics a positive line. Chemical degradation of the test strip can also occasionally lead to non-specific binding or a generalized background color change that may be misinterpreted as positive.
Next Steps After Using an Expired Test
Regardless of the result from an expired test—positive, negative, or inconclusive—the immediate next step is to retest using a new, non-expired device. It is advisable to wait a day or two before retesting to allow hCG levels, if present, to increase further and ensure the most accurate result. If the expired test showed a positive line, a new test is necessary to confirm pregnancy and rule out an evaporation line.
If the expired test showed a negative result, retesting is needed to rule out a false negative caused by degraded antibodies. Once a reliable, non-expired test confirms pregnancy, contact a healthcare provider to schedule initial prenatal care. If a period is missed but reliable, non-expired tests continue to show negative results, a consultation with a provider is appropriate to investigate potential underlying causes for delayed menstruation.