Antigen tests are a diagnostic tool designed to quickly identify active infections by detecting specific viral or bacterial components. These tests offer a rapid screening method, delivering results within minutes for immediate health assessments. They determine the presence of a pathogen when a person is actively infected. This helps individuals understand their infection status quickly, guiding decisions about isolation and further medical care.
The Nature of Antigens
Antigens are molecular markers, typically proteins or carbohydrates, found on the surface of pathogens like viruses or bacteria. These markers act as unique identifiers that the immune system recognizes as foreign substances. When foreign antigens enter the body, they can trigger an immune response, prompting the body to produce antibodies.
Rapid antigen tests commonly target specific proteins from viruses, such as the nucleocapsid protein of SARS-CoV-2, or components of bacterial cell walls. These tests look for these distinct molecular structures, which signal an active infection. The presence of these antigens indicates the pathogen is actively replicating in the body.
Mechanism of Antigen Tests
Antigen tests, often known as lateral flow immunoassays, operate on a “lock and key” principle, involving the binding of antigens to specific antibodies. A typical rapid antigen test strip contains a sample pad, a conjugate pad, a test line, and a control line. After a sample, such as a nasal swab, is collected and processed, it is applied to the sample pad.
The liquid sample moves along the test strip through capillary action, reaching the conjugate pad. This pad contains detection antibodies, often labeled with colored particles like gold colloid, that are specific to the target antigen. If target antigens are present, they bind to these labeled antibodies, forming antigen-antibody complexes. These complexes migrate along the strip to the test line, where immobilized antibodies capture them, leading to a visible colored line.
Understanding Test Outcomes
Interpreting antigen test results involves observing the appearance of lines on the test strip. A positive result shows two colored lines: one at the control (C) region and another at the test (T) region. A line at the test region indicates that specific antigens were detected in the sample, suggesting an active infection. Even a faint line at the test region should be considered a positive result.
A negative result occurs when a colored line appears only at the control (C) region, with no line visible at the test (T) region. This suggests the test did not detect target antigens, meaning either there is no active infection or the amount of pathogen is too low. The control line confirms the test worked correctly, indicating proper sample flow and reagent functionality. If the control line does not appear, the test is invalid and should be repeated.
Limitations and Reliability
Antigen tests offer speed and convenience, but they have limitations in reliability compared to other diagnostic methods, such as PCR tests. Sensitivity, the test’s ability to correctly identify positive cases, is a key factor. Antigen tests are less sensitive than PCR tests, meaning they may not detect an infection, especially early on or when the viral load is low, leading to potential false negatives. Studies have shown antigen test sensitivity for SARS-CoV-2 to be lower than PCR, sometimes as low as 47% compared to PCR as a reference.
The timing of the test is important, as antigen tests are most effective when a person has a high viral load, such as when symptoms are present or soon after onset. False positive results, though less common, can occur due to cross-reactivity with other substances or improper test conditions. The FDA recommends repeat testing after a negative antigen test, especially if symptoms are present or exposure is suspected, to reduce the risk of a missed infection. Antigen tests are useful for rapid screening, and a confirmatory PCR test may be recommended in certain situations.