The rapid antigen test (RAT) for COVID-19 uses lateral flow immunoassay technology housed inside a plastic cassette. Users are often confused by unexpected marks or very faint lines that appear on the result window. This uncertainty leads many to question whether these devices contain pre-existing marks, sometimes called “indent lines.” Understanding the test strip mechanics is essential for accurately interpreting results, especially when a line is not clearly visible.
The Standard Lines on a COVID Test Cassette
Lateral flow technology relies on a nitrocellulose membrane strip containing pre-applied reagents to detect the SARS-CoV-2 nucleocapsid protein. When the treated sample liquid travels up the strip via capillary action, it passes through two specific areas designed to display a colored line. These are the only two lines intended to appear during the test process.
The first line, labeled “C” for Control, is a procedural check confirming the test worked correctly. The C line contains antibodies that bind to a control material in the reagent buffer, ensuring the liquid sample flowed across the membrane as intended. If this line does not appear, the test is invalid, and the result cannot be trusted.
The second area is the Test line, labeled “T,” which detects the viral antigen. This line contains immobilized antibodies specific to SARS-CoV-2 proteins. If the sample contains the virus, viral antigens bind to colored nanoparticles and become trapped at the T line, creating a visible color change. The presence of a line in both the C and T zones defines a positive result.
Understanding Indent Lines and Ghost Marks
The concept of “indent lines” relates directly to the physical construction of the test strip membrane. An indent line is not a colored result but a physical feature—a faint, colorless, or slightly gray impression where the Test line reagents are applied. Manufacturers apply inactive, dried antibodies to this precise location to capture the virus.
Before the liquid sample is added, this dried, colorless strip of reagent may be faintly visible, especially under bright light or from a specific angle. This is the “indent line” or “ghost mark,” which is the physical track where the actual colored line would form if the test were positive. Since this is a structural element, any pre-existing mark lacking the specific color of the positive indicator (often pink, red, or purple) should be disregarded.
Ghost marks are distinct from a true positive result because they contain no captured viral antigen and no visible color from the nanoparticle conjugate. If a mark is visible before the test is run, or if it is a colorless indentation, it should be ignored. The mark’s sole function is to serve as the binding site for the colored complexes that signal a positive result. A valid result only occurs when the sample flow activates the reagents and deposits color at this location.
How to Interpret Faint or Delayed Lines
Any line appearing in the Test zone during the manufacturer’s specified reading window (typically 15 to 30 minutes) should be interpreted as a positive result. This rule holds true regardless of how faint the line appears. A faint line indicates that the test successfully detected the SARS-CoV-2 antigen, even if the viral load is relatively low.
The faintness of the line correlates with the concentration of viral protein in the sample. A bright, bold line suggests a high viral load. Conversely, a faint line may indicate an early infection, a resolving infection, or a less concentrated sample. In all cases, a faint line appearing within the time limit signifies the presence of the virus.
A different type of mark, known as an evaporation line, can appear if the test is read after the designated reading window has closed. As the liquid sample dries completely, residual components from the buffer or the sample itself can leave a faint, colorless, or grayish streak on the membrane. These evaporation lines form outside the valid reading time, do not indicate a positive result, and should be ignored, meaning the result is considered negative or invalid.