Rapid Antigen Tests and Their Uses

Rapid Antigen Test (RAT), sometimes called Rapid Antigen Test (RADT), Rapid Antigen Test (ART), or simply Rapid Test, is a rapid diagnostic test suitable for point-of-care testing that directly detects the presence or Antigen is absent.Such tests are a type of lateral flow test that detects antigens,which distinguishes them from other medical tests that detect antibodies (antibody tests) or nucleic acids (nucleic acid tests), whether they are laboratory types or point-of-care types. Rapid tests typically give results within 5 to 30 minutes, require minimal training or infrastructure, and offer significant cost advantages. [citation needed] Rapid antigen test for detection of SARS-CoV-2 (the virus that causes COVID-19) has been commonly used during the COVID-19 pandemic.Over the years, when referring to RAT or RADT, the early and main class of RAT—the rapid strep test for streptococci—has been referred to so often that the latter two terms are often treated roughly as synonyms for them. Since the COVID-19 pandemic, awareness of RAT is no longer restricted to health professionals, and COVID-19 has become an expected reference, thus requiring more precise use in other contexts.RAT is based on the principle of antigen-antibody interaction. Antibodies against the antigen of interest (usually a protein on the surface of the virus) are immobilized on the indicator line of the chromatography substrate and visualized (usually dye-labeled, although sometimes these antibodies are modified to fluoresce).When dissolved sample is added,it moves the visual marker, which then travels with the sample through the chromatography substrate.Viral particles (to which the antibody linked to the visualization marker has bound) are immobilized by the antibody immobilized on the indicator thread as they pass through the matrix.This also immobilizes the bound visual marker, allowing visual detection of significant levels of virus in the sample.Positive results of antigen tests should usually be confirmed by RT-qPCR or other tests with higher sensitivity and specificity. 

Uses

Common examples of RATs or RADTs include: 

•  Rapid tests associated with COVID-19 testing.

•  Rapid strep test (for strep antigen) .

•  Rapid Influenza Diagnostic Test (RIDT) (for influenza virus antigens).

• Malaria antigen detection test (Plasmodium antigen).

Science foundations and basic biology

RAT is an immunochromatographic assay that provides results visible to the naked eye (with or without special lighting, such as ultraviolet light).They are qualitative in nature,although there are limits to a rough order-of-magnitude estimate of viral load from the results. RAT is generally a screening test with relatively low sensitivity and specificity, so results should be evaluated against confirmatory tests such as PCR tests or Western blots.An inherent advantage of antigen tests over antibody tests (such as rapid HIV tests that detect antibodies) is that it may take a while for the immune system to produce antibodies after infection begins, but foreign antigens appear immediately. While false negatives are possible with any diagnostic test, this latency period can open up a particularly wide avenue for false negatives in antibody tests, although the specifics depend on the disease and test involved.A rapid antigen test typically costs about $5 to manufacture.

SARS-CoV-2 (Covid-19): rapid antigen test for diagnosis

For Rapid Detection of SARS-Cov-2 Virus in 15 Minutes During Covid-19 Infection in Nasopharyngeal Specimens.COVID-19 (coronavirus disease) is an infectious disease caused by the recently discovered coronavirus SARS-CoV-2 (2019-nCoV).Although RT-qPCR is the official technique for diagnosing COVID-19, other methods are available such as serological tests,rapid saliva tests, and antigen tests.Antigen tests are the latest tests developed to diagnose COVID-19.They are rapid tests designed to detect antigens of the SARS-CoV-2 virus,which are proteins that are present on the surface of the virus.Like the RT-qPCR test, this test is performed on nasopharyngeal swab specimens.However, while antigen assays are less sensitive than RT-qPCR assays, they have the advantage of being faster (15-30 minutes) and less expensive.Also,like serological tests,they don't require any special equipment.Antigen rapid detection,like serological detection, is based on the principle of immunochromatography.Antibodies against SARS-Cov-2 viral antigens (usually the nucleocapsid protein or N protein) will detect the virus in nasopharyngeal samples.Positive results for antigen testing need to be confirmed by RT-qPCR testing.

What are the advantages of a quick test:

• Rapid results within 15 minutes.

• Can be detected without any specific equipment.

• Ease of use: 3-4 step simple protocol, no sample pretreatment required.

•  Efficient case tracing: Immediate results allow for earlier initiation of contact tracing and isolation.

• Compatible with nasopharyngeal and oropharyngeal swabs.

• Store at room temperature.

What is the rationale for the rapid antigen test used to diagnose COVID-19?

The COVID-19 Rapid Antigen Test detects viral antigens of the SARS-CoV-2 virus by visual interpretation of colored lines.Anti-SARS-CoV-2 antibodies are immobilized on the test area of the nitrocellulose membrane. Anti-SARS-CoV-2 antibodies bound to colored particles are immobilized on binding buffer.The specimen is added to an extraction buffer optimized to release antigens from the SARS-CoV-2 virus present in the specimen.During the test, the extracted antigen binds to the anti-SARS-CoV-2 antibody bound to the stained particles. As the sample migrates along the strip by capillary action and interacts with the reagents on the membrane, the complex is captured by the anti-SARS-CoV-2 antibody in the test area.The presence of colored bands in the test area indicates a positive result for SARS-CoV-2 viral antigen and the absence indicates a negative result.The colored band in the control area serves as a program control, indicating that the appropriate sample volume has been added and the membrane is running.