Rapid antigen COVID-19 tests have become a widely used tool for at-home detection of the SARS-CoV-2 virus. These convenient tests offer a quick way to determine if viral proteins are present in a sample, helping individuals make informed decisions about their health and potential for transmission. Understanding the different indicators on these tests is important for accurate interpretation.
The Control Line Explained
The “C” on a rapid antigen COVID-19 test stands for the Control line and indicates test validity. It confirms the test functions correctly and the sample moved properly through the test strip by capillary action. Without a visible Control line, any other result is invalid.
The Control line is formed by specific antibodies immobilized on the test strip that capture control reagents. These control reagents are designed to bind to antibodies that are part of the test’s internal system, confirming the liquid sample has saturated the strip and the chemical reactions are occurring. Thus, a visible “C” line signifies proper test procedure and working components.
Decoding Your Test Results
Interpreting rapid antigen test results involves observing both the Control (C) and Test (T) lines. The “T” line indicates the presence of SARS-CoV-2 antigens, which are specific proteins from the virus. The appearance of this line depends on a reaction between viral antigens in the sample and specific antibodies embedded in the test strip.
If only the Control (C) line appears, with no Test (T) line, the result is negative. This indicates SARS-CoV-2 antigens were not detected or were below the test’s detection threshold. A negative result does not completely rule out an infection, especially if symptoms are present, and repeat testing may be recommended.
A positive result shows both the Control (C) and Test (T) lines. Both lines suggest SARS-CoV-2 antigens were detected, indicating likely COVID-19. Even a faint T line should be interpreted as a positive result, as its intensity does not change the result’s meaning.
An invalid result occurs if the Control (C) line does not appear, regardless of a T line. This signifies the test did not run correctly, possibly due to insufficient sample volume, improper test procedure, or a defective test kit. In such cases, the test should be discarded, and a new test should be performed with a fresh sample and a new device.