Home rapid antigen tests use lateral flow immunoassay technology to quickly check for COVID-19 infection, offering results in minutes. Users are often confused by faint lines, leading to questions about evaporation lines that can mimic a positive result. Understanding the science and strict reading protocols is necessary to accurately interpret results that are not clearly negative. This clarity helps in making informed decisions about health and isolation.
The Mechanism of Evaporation Lines
The COVID-19 rapid antigen test is a specialized lateral flow assay (LFA) designed to detect SARS-CoV-2 viral proteins (antigens). When the nasal sample is added, the liquid flows across a membrane strip by capillary action. This liquid carries tiny, colored particles, often gold nanoparticles, which are bound to antibodies designed to attach to the target viral proteins.
An evaporation line occurs in any LFA device after the liquid solvent has fully dried. As water evaporates from the porous membrane, it leaves behind a visible, non-specific trace of residual components, such as unbound antibodies or proteins. This trace appears where the positive test line (T-line) forms but lacks the colored nanoparticles needed for a true binding reaction. Since this trace is a physical residue, not a chemical reaction, it is often mistaken for a faint positive result.
Valid Test Reading Protocol
The most determinative factor distinguishing a valid line from an evaporation line is the manufacturer’s specified reading window. Rapid antigen tests are engineered to provide a chemically reliable result within a strict timeframe, typically 15 to 30 minutes after the sample is added. This window allows time for the sample to flow, the reaction to occur, and the colored control line (C-line) to confirm the test is functional.
Any line appearing after this designated time window must be disregarded as an invalid result. Once the test membrane dries completely, the chemical environment changes, and the color development process becomes unreliable. Interpreting a line that appears later is a misinterpretation of the test’s design, as the integrity of the test depends on the precise flow of liquid and timing.
Distinguishing Evap Lines from True Positives
A true positive line, even a faint one appearing within the valid reading window, always contains the colored reagent, such as the pink or red dye from the gold nanoparticles. Faintness indicates a low concentration of the SARS-CoV-2 viral protein (low viral load). This low viral load may be due to early or late stages of infection, or inadequate sample collection, but it still signifies a positive result.
In contrast, an evaporation line is typically translucent, colorless, or a hazy gray trace lacking the distinct saturation of the colored reagent. This ghost line often appears blurry or smeared, rather than a clean, precise band of color. If the line appeared within the valid time window, the presence of any true color means the test should be treated as positive. If the line is only visible after the time limit and is a gray shadow, it is most likely an evaporation line.
Next Steps After an Ambiguous Result
If a test result is ambiguous, such as a faint colored line appearing near the end of the valid reading window, the most appropriate first action is to retest. Use a new kit, preferably from a different manufacturer, and ensure the sample collection process is meticulous to maximize the chance of a clear result. Waiting 24 to 48 hours to retest is also a sound strategy, especially if symptoms are new, as the viral load may increase during that time.
If uncertainty remains after serial rapid testing, a confirmatory molecular test, such as a PCR test, can be sought. PCR tests are more sensitive and can detect lower levels of the virus. Regardless of visual ambiguity, if there is a strong suspicion of infection due to symptoms or exposure, public health guidelines for isolation should be followed until a definitive negative result is obtained.