Rapid antigen tests are common tools for detecting SARS-CoV-2, the virus responsible for COVID-19. These at-home tests provide quick results, helping individuals make informed decisions about their health. Understanding the “C” and “T” lines on the test strip is important for accurate interpretation.
The Control Line (‘C’)
The “C” line, or “Control” line, serves as an internal quality check to ensure the test functions correctly. This line should always appear, regardless of whether SARS-CoV-2 viral proteins are present. Its appearance confirms the sample flowed properly and the test components are working as intended. If the “C” line does not appear, the test is invalid, meaning the result cannot be trusted. This absence can indicate improper use, insufficient sample volume, or a faulty or expired test kit.
The Test Line (‘T’)
The “T” line, or “Test” line, indicates the presence of SARS-CoV-2 viral proteins (antigens) in the collected sample. These antigens bind to specific antibodies on the test strip, creating a visible colored line. Even a faint “T” line should be interpreted as a positive result. The “T” line’s appearance is contingent upon the “C” line also being present, confirming the test’s validity.
Interpreting Your Results
Interpreting the “C” and “T” lines is essential for understanding your rapid COVID-19 test outcome.
Positive Result
A positive result is indicated when both the “C” line and the “T” line appear, even if the “T” line is faint. This suggests a current SARS-CoV-2 infection, and the individual is likely contagious.
Negative Result
A negative result is displayed when only the “C” line appears, with no visible “T” line. This means SARS-CoV-2 antigens were not detected in the sample, though it does not completely rule out an infection, especially if the viral load is low.
Invalid Result
An invalid result occurs if the “C” line does not appear. This means the test did not function properly, and the result is inconclusive. A new test kit should be used, carefully following all instructions.
Test Reliability and Next Steps
Rapid antigen tests are generally effective for detecting COVID-19, particularly when an individual has a high viral load and is most infectious. However, they may produce false negatives, especially early in an infection or in asymptomatic individuals, as they require a higher concentration of the virus to trigger a positive result compared to PCR tests.
Positive Result Actions
If you receive a positive result, it is advisable to isolate, consult a healthcare provider for guidance, and inform close contacts.
Negative Result Actions
For a negative result, especially if symptoms are present or there was a known exposure, monitoring symptoms and considering retesting within 24-48 hours is recommended.