Rapid COVID-19 antigen tests have become a common tool for quickly checking for the presence of the SARS-CoV-2 virus. These tests are designed for ease of use at home and provide results within minutes, typically ranging from 5 to 30 minutes. The results are displayed by lines that appear next to the letters ‘C’ and ‘T’ on the test cassette, each indicating a specific aspect of the test’s performance or outcome.
Understanding the Control Line
The ‘C’ on a rapid antigen test stands for the Control line, which acts as an internal quality check to confirm the test is working correctly. This line should always appear, regardless of whether SARS-CoV-2 antigens are detected in the sample. Its presence indicates that the test strip has been properly saturated with the sample liquid and that the test components, including the reagents and antibodies, are functioning as intended. If the control line does not appear, the test result is considered invalid, even if a line appears next to the ‘T’. An absent control line suggests issues such as insufficient sample volume, improper test procedure, or a defective test kit.
Understanding the Test Line
The ‘T’ on a rapid antigen test indicates the Test line, and its appearance signifies the potential presence of SARS-CoV-2 viral proteins, known as antigens. This line contains specific antibodies designed to bind to these viral antigens if they are present in the collected sample. When the sample flows across the test strip, any SARS-CoV-2 antigens present will bind to the colored nanoparticles and then to the immobilized antibodies at the ‘T’ line, forming a visible colored line. The test line will only become visible if a sufficient concentration of viral antigens is detected in the sample. The intensity of the ‘T’ line can vary, appearing faint or dark, depending on the amount of viral load in the sample. However, any visible line at the ‘T’ position, regardless of its faintness, indicates a positive result.
Interpreting Your Test Results
Interpreting the results of a rapid COVID-19 antigen test involves observing the presence or absence of both the Control (‘C’) and Test (‘T’) lines within the specified timeframe, usually 15 to 30 minutes. There are three main outcomes to consider when reading your test.
Negative Result
A negative result is indicated by the appearance of only the Control line (‘C’), with no line visible at the Test position (‘T’). This outcome suggests that SARS-CoV-2 antigens were not detected in the sample at a level sufficient for the test to register, or that no antigens were present.
Positive Result
A positive result is shown when both the Control line (‘C’) and the Test line (‘T’) are visible. The appearance of both lines indicates that SARS-CoV-2 antigens have been detected in the sample, meaning the individual is likely infected with the virus and potentially contagious.
Invalid Result
An invalid result occurs if the Control line (‘C’) does not appear, regardless of whether a line is present at the Test position (‘T’). In such cases, the test result cannot be relied upon, and it is necessary to discard the invalid test and perform a new test with a fresh kit and sample.