Rapid antigen COVID-19 tests are a widely accessible tool for quickly checking for SARS-CoV-2 infection. Understanding how to properly interpret these results is important for personal health decisions and public health measures.
Understanding the Control Line
The “C” on a rapid antigen test stands for “Control.” This line serves as an internal quality check, confirming that the test was performed correctly and that its components are working as intended. The control line should always appear, regardless of whether viral antigens are present in the sample. Its appearance indicates that sufficient sample volume was added and that the liquid migrated properly through the test strip. If the control line does not appear, the test is considered invalid and must be repeated with a new kit.
Understanding the Test Line
The “T” on a rapid antigen test stands for “Test.” This line indicates the presence of specific viral proteins, known as antigens, from the SARS-CoV-2 virus in the collected sample. The test works by using specialized antibodies that bind to these viral antigens. If SARS-CoV-2 antigens are present, they form a complex with these antibodies, resulting in a visible line at the “T” region.
The appearance of the “T” line suggests an active infection, as it detects the viral proteins produced during replication. The intensity of the “T” line can vary; a strong, dark line typically indicates a higher concentration of viral material, while a faint line suggests a lower viral load. However, any visible line at the “T” position, no matter how faint, should be interpreted as a positive result.
Interpreting Your COVID Test Results
Interpreting rapid antigen test results requires observing both the control (C) and test (T) lines. Results are typically read within 15 to 30 minutes; readings taken after this window may not be accurate.
Negative Result
A negative result occurs when a line appears only in the “C” region, with no line visible at the “T” position. This indicates that no viral antigens were detected at the time of testing. A negative result does not completely rule out infection, especially if tested early in the infection cycle or if the viral load is low. If symptoms are present or exposure is known, repeat testing after 48 hours is often recommended.
Positive Result
A positive result is indicated by the presence of both a “C” line and a “T” line, regardless of how faint the “T” line may be. This signifies that SARS-CoV-2 antigens have been detected, making it likely that an active infection is present. Upon receiving a positive result, assume you have COVID-19 and follow public health guidelines, such as isolating to prevent further spread and contacting a healthcare provider for next steps.
Invalid Result
An invalid result occurs if the “C” line does not appear, or if no lines are visible. This means the test did not function properly and cannot be relied upon. Reasons include improper sample collection, insufficient liquid, or a faulty test kit. If invalid, discard the test and perform a new one using a fresh kit, carefully following instructions.