16.2.5: COVID-19 Rapid Antigen Tests
- Page ID
- 122733
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The various COVID-19 rapid antigen tests are also examples of lateral flow immunologic assays. They are qualitative serologic tests for detecting COVID-19 infection. They detect the nucleocapsid protein antigen (N protein) of SARS-Cov-2, the virus that causes COVID-19. Swabs from the nose or throat are placed in a buffer solution that maintains a pH of 7.4, the pH of the blood. The buffer solutions also contain agents that lyse the viruses, releasing the various viral antigens, including the N protein antigen. The N protein, the unknown antigen for which one is testing, is identified in the sample by using known monoclonal antibodies, made in animals such as rabbits, mice, or chickens, against the N protein and bound to colloidal gold, which is red in color.
Like the Strep A and pregnancy tests mentioned above, this test uses a color immunochromatographic assay to detect the antigen-antibody reaction. The test consists of a membrane strip that is precoated with known anti-N protein antibody-colloidal gold conjugate (known antibodies made in rabbits, mice, chickens, etc. against the N protein of SARS-Cov-2 with red colloidal gold particles attached) located in a pad at the beginning of the strip. In this example discussed here, the anti-N protein antibodies are made using monoclonal antibody technique in rabbits. It is also precoated with known anti-N protein antibody (known antibodies made in rabbits against N protein without attached red colloidal gold) that are immobilized at the test line where the test results are read. The red colloidal gold particles attached to the anti-N protein antibodies is what ultimately causes the positive red band in the test area.
When the patient's sample is added to the sample well, the N-protein antigen, if present, begins to move chromatographically up the membrane and binds to the red-colored known anti-N protein rabbit antibody-gold conjugate in the pad located at the beginning of the test assembly, forming a N protein antigen-antibody-gold conjugate complex. This N protein antigen-antibody complex continues to move up the membrane to the test line region where the immobilized known rabbit anti-N protein antibodies are located.
If N protein is present in the patient's sample, a red-colored sandwich of N protein antigen/anti-N protein rabbit antibody-gold conjugate forms in the test line region of the strip resulting in formation of a red band.
The control region of the strip is precoated with immobilized anti-rabbit antibody antibodies (antibodies made in a different animal against rabbit antibodies). The red color at the control line region appears when molecules of the anti-N protein antibody-red latex conjugate that were not trapped at the test line reach the control area and are stopped by binding to the anti-rabbit antibody antibodies. The red color indicates that the test is finished. As a result, a positive test for COVID-19 antigen appears as a red band in the test result area and a red band in the control area.
If there is no detectable SARS-Cov-2 N protein antigen present in the patient's sample, no red band appears in the test result region of the strip and a single red band appears in the control line region only, indicating a negative antigen test for COVID-19.