The widespread availability of rapid COVID-19 antigen tests has made self-testing common practice for determining infection status. These convenient kits allow individuals to quickly check for the presence of SARS-CoV-2 proteins from a nasal swab sample. The accuracy of this tool relies heavily on correct execution and accurate interpretation of the result window. Understanding the visual cues provided by the test strip is paramount to knowing whether to take precautions or seek further medical advice.
Understanding the Control and Test Lines
A rapid antigen test utilizes a lateral flow immunoassay, a technology designed to detect specific viral proteins known as antigens. The test cassette typically features a rectangular result window displaying two distinct markers, labeled “C” and “T.” The “C” stands for the Control line, which is a procedural quality check that ensures the test has run correctly.
The Control line must appear for the result to be considered valid, confirming that the liquid sample successfully flowed through the test strip materials. The “T” designates the Test line, and its appearance indicates the presence of SARS-CoV-2 antigens in the sample. These antigens bind to antibodies coated on the test strip, creating a visible colored line.
Reading Positive, Negative, and Invalid Results
Interpreting the combination of lines determines the final outcome. A result is considered positive if both the Control (C) line and the Test (T) line are visible. The appearance of the T line means the test detected viral antigens, indicating a likely infection with COVID-19.
Any visible line at the T position, even one that is faint, must be read as positive. While the intensity of the T line may correlate with the viral load present, a faint line still signifies a positive test.
A negative result is indicated when only the Control (C) line appears, and there is no visible line at the Test (T) position. This means viral antigens were not detected in the sample. However, a negative result does not completely rule out an infection, especially if symptoms are present.
A test is considered invalid if the Control (C) line does not appear, regardless of whether the T line is present. The absence of the C line signifies a failure in the test procedure, perhaps due to improper sample collection or insufficient liquid flow. An invalid result is inconclusive, meaning a new test must be performed immediately with a fresh kit.
Why Test Reading Time Matters
The time between applying the sample and reading the result is a factor in ensuring accuracy. Every manufacturer specifies a particular reading window, often between 15 and 30 minutes, which must be followed. Reading the test too early risks a false negative, as the chemical reaction may not have had enough time to complete.
Reading the test too late, typically past the 30-minute mark, compromises the result’s integrity. Over time, the test strip components can continue to interact, potentially leading to the appearance of non-specific lines. These misleading lines can be misinterpreted as a positive result, rendering the late reading inaccurate.
Next Steps After Your Result
The immediate steps following a test result depend on the outcome displayed. If the test is positive, the individual should assume they have COVID-19 and follow local public health guidance for isolation. This typically involves staying home for a specified number of days and notifying close contacts of the exposure.
A negative result means the virus was not detected, but caution should not be disregarded, particularly for symptomatic individuals. Because antigen tests are less sensitive than laboratory-based tests, a negative result should be followed by a repeat test 48 hours later to confirm the status. If the test is invalid, the kit must be discarded and the procedure repeated using a brand-new testing device.