COVID-19 antigen tests detect an active SARS-CoV-2 infection by identifying specific viral proteins, known as antigens. These tests offer a quick and convenient method for preliminary screening, providing results within minutes. Unlike laboratory-based tests that detect viral genetic material, antigen tests are designed for ease of use outside of a clinical laboratory setting.
The Science Behind the Test
COVID-19 antigen tests operate on a principle called a lateral flow immunoassay. This technology utilizes a paper-based platform to detect chemical analytes rapidly. When a prepared sample is applied to the test strip, it begins to migrate through the strip via capillary action.
As the sample moves, it encounters a region containing specialized antibodies conjugated to a visualization component, typically gold nanoparticles. If SARS-CoV-2 antigens are present in the sample, they bind to these mobile, gold-conjugated antibodies, forming antigen-antibody complexes. These complexes then continue to migrate along the strip until they reach a designated test line.
The test line contains immobilized antibodies specific to the SARS-CoV-2 antigens. When the antigen-antibody complexes reach this line, they create a visible colored line, indicating a positive result. A separate control line always appears to confirm the test is working correctly, regardless of the presence of the virus.
Components of an Antigen Test Kit
A typical COVID-19 antigen test kit contains several components. It usually includes a sterile nasal swab, used for collecting a sample from the nasal cavity. This swab gathers cells and mucus that may contain viral particles.
After collection, the swab is agitated in a small tube containing a buffer solution. This solution helps to suspend the collected sample and can disrupt virus particles, releasing their antigens. The processed liquid is then applied to the test cassette.
The test cassette is a plastic housing that encases the lateral flow test strip. It has a designated well for applying the sample and a window to view the results.
Interpreting Test Results
A valid test always displays a colored line at the control (C) region. This control line confirms the test functioned properly and that enough sample migrated through the strip. If the control line does not appear, the test is invalid and should be discarded.
A positive result shows two colored lines: one at the control (C) region and another at the test (T) region. The presence of a line, however faint, at the test (T) region signifies a positive result, indicating a likely active infection.
A negative result shows only one colored line, appearing solely at the control (C) region. The absence of a line at the test (T) region suggests that the concentration of SARS-CoV-2 antigens in the sample was below the detection threshold. A single negative antigen test does not completely rule out an infection, especially early in the course of infection.
Factors Influencing Test Reliability
Several factors can influence antigen test accuracy. The timing of the test relative to symptom onset or exposure plays a significant role. Antigen tests are generally most reliable when the viral load is high, which typically occurs during the acute phase of infection. Testing too early or too late in the infection can lead to a lower chance of detection.
Proper sample collection technique is also important for test reliability. Inadequate swabbing, such as not collecting enough material from the nasal passages, can result in an insufficient amount of virus for the test to detect. Adhering to the manufacturer’s instructions for sample collection and preparation helps ensure the test can perform as intended.
The concentration of the virus in an individual’s sample, known as viral load, directly impacts the test’s sensitivity. Tests are more likely to detect the virus when viral loads are high. If the viral load is low, particularly in asymptomatic individuals or those early in infection, the test may produce a negative result even if the person is infected. Repeating a negative test after 48 hours is often recommended, especially if symptoms are present.