The color of the lines on a rapid drug screen (dip card or cup test) provides a quick visual result to determine if certain substances are present in a sample. These on-site tests are designed for preliminary screening, offering a qualitative result that indicates if a substance is present or absent above a specific concentration level. Unlike laboratory instruments that measure exact quantities, these tests rely on chemical reactions that produce colored lines for interpretation.
The Science Behind Color Indicators
The rapid drug test employs a technology known as a lateral flow immunoassay, which is based on the principle of competitive binding. When the sample is introduced, it flows up a porous membrane strip, carrying colored tracer particles, most often made from colloidal gold. These nanoparticles naturally appear red, but manufacturers can modify them to display other colors like blue or green for differentiation.
The drug molecules in the sample compete with a fixed quantity of drug-like molecules immobilized on the “Test” line location. The colored tracer particles are bonded to antibodies specific to the target drug. If the drug is present, it binds to these antibodies, preventing them from attaching to the fixed drug molecules at the Test line.
This competition for the limited number of antibody binding sites is what determines the final result. If there is a high concentration of drug in the sample, it saturates the antibodies, and they are carried past the Test line without binding there, leaving no color to form a line. Conversely, if no drug is present, the colored antibodies are free to bind to the fixed molecules on the Test line, resulting in a visible colored line.
Ensuring Test Validity with the Control Line
Every rapid drug test incorporates a Control Line (C) as an internal check to ensure the assay functioned correctly. The appearance of a colored line at the C position is required for a valid test result. The color of this line is determined by the dye used and does not affect its function.
The Control Line is coated with general antibodies that capture the excess colored tracer particles that flowed past the Test area, regardless of whether a drug was present. Its appearance confirms that sufficient sample volume was added and that the chemical reagents in the test strip were active and flowed correctly across the membrane.
If the Control Line does not appear within the specified reading time, the test is considered “Invalid.” This means the test failed due to procedural error, such as insufficient sample or a defect in the test kit. Any visible line in the Test area is disregarded, and the screening procedure must be repeated using a new device.
Interpreting Positive and Negative Results
The interpretation of the Test Line (T) is based solely on the presence or absence of a colored line. The specific hue of the dye (red, green, or other) is the pigment chosen by the manufacturer for visibility, meaning the same interpretation rules apply regardless of the color.
A Negative Result is indicated by the appearance of two lines: the Control Line (C) and the Test Line (T). This means the drug concentration is below the established cut-off level. Even a faint line in the Test region is interpreted as a negative result, regardless of its intensity.
A Preliminary Positive Result is indicated when only the Control Line (C) appears, with a complete absence of a line at the Test Line (T). The lack of a T line means the drug was present at a concentration equal to or greater than the test’s cut-off level. This high concentration of drug bound all the colored antibody particles, preventing them from binding at the Test line.