What is antigen

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Cancer Prevention Research. Cancer Treatment Research. Cancer Health Disparities. Childhood Cancers Research. Global Cancer Research. Cancer Research Infrastructure. In the last few years, therapeutic antibodies have become the main class of new drugs in development, and by December , 79 therapeutic monoclonal antibodies had been approved by the US FDA. The range of conditions they can be used to treat includes several types of cancers , autoimmune conditions, and infectious diseases such as Zika.

A number of studies are currently investigating their potential as a treatment for COVID , including via convalescent plasma therapy. Therapeutic antibodies work by binding with high specificity to the target antigen and stimulating an immune response, which may involve inhibition of ligand binding or tagging the cell for binding by cytotoxic T cells.

Antibodies can be produced using a variety of techniques, including hybridoma technologies, transgenic mice, and in vitro display technologies. Clinical performance of NAATs and antigen tests may differ from clinical utility when considering issues of test availability, quality of specimen collection and transport, and turnaround times of results. Based on their instructions for use, some point-of-care NAATs may not be used for confirmatory testing.

NAATs that generate presumptive results are not appropriate for use in confirmatory testing. The sensitivity of antigen tests varies but is generally lower than most laboratory-based NAATs. This may result in a negative antigen test result, while a more sensitive test, such as most NAATs, may return a positive result.

Studies external icon have shown that antigen tests have comparable sensitivity to laboratory-based NAATs when viral load in the specimen is high and the person is likely to be most contagious.

Despite the high specificity of antigen tests, false positive results will occur, especially when used in communities where the prevalence of infection is low — a circumstance that is true for all in vitro diagnostic tests. In general, for all diagnostic tests, the lower the prevalence of infection in the community, the higher the proportion of false positive test results. Positive and negative predictive values of all in vitro diagnostic tests e.

Pretest probability considers both the prevalence of the target infection in the population that is being tested as well as the clinical context of the individual being tested.

If the prevalence of infection in the community is high, and the person being tested is symptomatic, then the pretest probability is generally considered high. If the prevalence of infection in the community is low, and the person being tested is asymptomatic and has not had any known contact to a person with COVID, then the pretest probability is generally considered low.

To help estimate pretest probability, CDC recommends that laboratory and testing professionals who perform antigen testing determine infection prevalence based on a rolling average of the positivity rate of their own SARS-CoV-2 testing over the previous 7—10 days. Additionally, state health departments generally publish COVID data on testing positivity rates and case rates for their communities. For example, the performance of antigen tests can be affected if the test components are not stored and handled properly.

The package insert for antigen tests also includes instructions about how to read the test results, including the appropriate time to read the results and whether the results should be interpreted visually or with an instrument analyzer. Reading the test before or after the specified time could result in false positive or false negative test results. Processing multiple specimens successively or in batch mode may increase the risk of contamination and may make it more challenging to ensure that each specimen is incubated for the correct amount of time before the result is read.

Refer to the package insert for the correct incubation time for that test, and then monitor and ensure proper timing for each specimen during testing and when reading results. All testing for SARS-CoV-2, including antigen testing, depends on the integrity of the specimen, which is affected by procedures for both specimen collection and handling.

Improper specimen collection, such as swabbing the nostril too quickly, may cause insufficient specimen collection, resulting in limited amounts of viral genetic or antigenic material for detection. Time from specimen collection to testing should be minimized, and the temperature of the specimen during this time must be controlled. Quality assurance procedures should be followed to prevent cross-contamination and inaccurate test results. If antigen testing returns multiple unexpected positive results, it may be appropriate to stop testing patient specimens, review all procedures, disinfect all surfaces, change gloves, and run control specimens before restarting the testing of patient specimens.

In such circumstances, confirmatory testing should be considered for people who received unexpected results, regardless of pretest probabilities. Clean work surfaces and equipment regularly daily or as often as needed with soap or detergent.

Gloves should be changed before collecting, handling, and processing a new specimen in the antigen test system. Failing to change gloves can increase the risk of cross-contamination and false antigen test results. Some antigen tests have explored the use of viral transport medium VTM during specimen collection, but the use of VTM may cause false test results from either cross-reactivity with the capture antibodies or dilution of the specimen that decreases the sensitivity of the test.

Laboratories and testing sites should refer to the instructions for use and the package insert that are specific for the test that they are using regarding the use of VTM.

Evaluating the results of an antigen test for SARS-CoV-2 depends primarily on the clinical and epidemiological context of the person who has been tested e. For additional details on testing recommendations see guidance for fully vaccinated people. A particularly important aspect of epidemiological context is whether the person to be tested is a resident or an employee of a congregate living facility.

In addition, evaluating the results of an antigen test for SARS-CoV-2 should consider the performance characteristics e. The evaluation of an antigen test result should consider whether the person has experienced symptoms, and if so for how long. Generally, healthcare providers can rely upon a positive antigen test result for a symptomatic patient because the specificity of current FDA-authorized antigen tests is high.