Rapid Antigen Tests
Last update: September 11, 2020.
What are rapid antigen tests?
Antigen tests detect proteins that are structural or functional components of the SARS-CoV-2 virus and therefore highly specific for that pathogen.
In this sense, rapid antigen tests for SARS-CoV-2 provide an immediate response ―between 15 and 30 minutes after taking the nasopharyngeal sample―, qualitative (yes or no), on the presence of the pathogen in the sample of the patient, and are ideal to be performed as rapid tests at the client’s home.
In addition, as can be seen in the following image, its suitability window is especially indicated to detect a SARS-CoV-2 infection even before symptoms begin, unlike antibody tests ―such as IgM or IgG―, which cannot be used until 14 days from the onset of symptoms, since the body does not start to generate these antibodies until after those days.
Uses and purposes
Rapid antigen tests can be used in a variety of testing strategies to respond to the coronavirus disease 2019 (COVID-19) pandemic, but they can all be grouped into two broad groups: diagnostic tests and screening tests.
Diagnostic tests for SARS-CoV-2 are intended to identify current infection in people and are performed when a person has signs or symptoms consistent with COVID-19, or when a person is asymptomatic but has a known recent exposure or suspected to SARS-CoV-2.
Screening tests for SARS-CoV-2 are intended to identify infected persons who are asymptomatic and have no known or suspected exposure to SARS-CoV-2. Examples of screening include tests in collective settings, such as a workplace ―which evaluates its employees―, a school ―which evaluates its students, teachers, and staff―, a long-term care facility, or a correctional facility, among others.
Only 15 minutes are required to obtain the result at the sample collection site, without the need to transfer the samples to the laboratory.
With sensitivity and specificity values greater than 90 and 95%, respectively, comparable to those of RT-PCR (the gold standard).
The cost of nursing, travel, testing, positive management, waste management and the nCoVPASS certificate is between 3 and 4 times lower than RT-PCR.
It is important that healthcare professionals understand the performance characteristics ―including analytical sensitivity and specificity― of the particular rapid antigen test being used, and that these follow the manufacturer’s instructions and the leaflet insert. Sometimes it may be necessary to confirm the result of a rapid antigen test with a nucleic acid test like PCR.
When rapid antigen tests are used for diagnosis in symptomatic persons within the first five to seven days of the onset of symptoms, the confirmatory PCR test may not be necessary.
When these are used for screening tests, the positive result should be considered presumptive. In this sense, people who receive a positive rapid antigen test should be isolated and go immediately to their health center to be confirmed by a PCR. On the other hand, a negative rapid antigen test used as a screening test can be considered definitely negative.