Rapid COVID-19 antigen tests are widely accessible for at-home screening, providing quick results for potential SARS-CoV-2 infection. These tests function by detecting specific proteins, known as antigens, from the SARS-CoV-2 virus. Understanding how to correctly interpret the results of these rapid tests is important for individuals to take appropriate action and help manage the spread of the virus. These convenient kits typically deliver results within 15 to 30 minutes, allowing for prompt decision-making.
The Control Line’s Purpose
The “C” on a rapid COVID-19 test stands for the Control line. It serves as an internal quality check, ensuring the test functions as intended. A visible line at the “C” position confirms that the test procedure was performed correctly, the sample migrated properly across the test strip, and the test reagents are active. This line should always appear, regardless of viral antigens, indicating the test’s procedural integrity. The control line typically contains pre-applied antibodies that capture gold nanoparticles, which are part of the test components, confirming that the liquid sample has moved through the strip effectively.
If the Control line does not appear, the test result is considered invalid. An absent “C” line suggests a malfunction, due to insufficient sample volume, incorrect procedural techniques, or faulty test components. In such cases, the test should be discarded, and a new test should be performed with a fresh kit, carefully following the manufacturer’s instructions. The presence of the control line is a fundamental indicator that the test is reliable and its results can be trusted.
Interpreting Your Test Results
Interpreting a rapid antigen test involves observing both the “C” (Control) line and the “T” (Test) line. A positive result is indicated by the appearance of two colored lines: one at the “C” and another at the “T”. This signifies the detection of SARS-CoV-2 antigens in the sample, meaning the individual is likely infected with the virus. Even a faint line at the “T”, alongside a visible “C” line, should be interpreted as a positive result.
A negative result shows only one colored line at the “C”, with no line at the “T”. This indicates that the test did not detect SARS-CoV-2 antigens. While a negative result suggests no detectable infection, it does not entirely rule out COVID-19, especially in the early stages of infection or if viral load is low, making false negatives possible. If neither the “C” nor the “T” lines appear, or if only the “T” line appears without the “C” line, the test is invalid, and no conclusion can be drawn.
What to Do After Testing
If a rapid antigen test is positive, isolate at home to prevent transmission. Public health guidelines often recommend isolating for 5 days from the day symptoms began or the test was taken. Individuals should also consider contacting a healthcare provider for guidance and inform close contacts about their positive status.
For a negative result, remain vigilant, especially if symptoms are present or known exposure. Retesting is often recommended if symptoms persist, with some guidelines suggesting serial testing to increase accuracy. If the test result is invalid, the most immediate step is to repeat the test using a new kit and carefully review the instructions to ensure proper execution.