To diagnose and contain the spread of the coronavirus, testing is essential. There are two types of tests covid-19: those designed to detect if you have the infection now or those designed to check if you have previously been infected by the virus, SARS-CoV-2, that causes the disease.
Like any other product, these tests have varying degrees of accuracy and reliability, and can be used to achieve different goals.
Are there false positives in antigen tests?
We speak of a false positive when the test result indicates the presence of the virus without the person being infected.
The RT-PCR technique is highly specific (99.5%), so the probability of a false positive is very low. It is also very sensitive, so much so that it allows the virus to be detected between 3 and 5 days after acquiring the infection and even weeks later. Although this viral RNA test does not always indicate current infection, it does not differentiate between active infection and resolved infection.
Antigen tests, like NAATs, are highly specific. They are comparable to the RT-PCR technique in the detection of SARS-CoV-2 infection in the first week after infection. In the nasopharynx, the highest viral load is observed mainly in the first 5 to 7 days and then decreases until it disappears. After this period of time, RT-PCR has a higher diagnostic sensitivity than the antigen test.
False positives in antigen tests are more frequent in mass screening in populations with a low prevalence of infection. They can also be due to sample contamination.
The factors that minimize the possibility of false positives in diagnostic tests are a high prevalence of infection in the community, the presence of symptoms suggestive of covid-19 in the person, and a high specificity of the test.
Confirm the result of an antigen test
The result of the antigen test must be confirmed by NAAT depending on the type of case and the probability of infection. It is considered that there is a high probability of infection if there is contact with a confirmed or suspected positive case and the person is not fully vaccinated or has not had a SARS-CoV-2 infection in the last 3 months.
In the community setting, in a person with symptoms suggestive of covid-19 of 5 days or less of evolution, a professional positive antigen test confirms the infection.
However, the health authorities may consider the results of the self-diagnosis tests to establish isolation and control measures.
If the person is symptomatic, it should be confirmed by RT-PCR
In the case of a negative antigen test, if the person is symptomatic, it should be confirmed by RT-PCR, especially if the symptoms began more than 5 days earlier, due to the decreased sensitivity of the antigen test. Also in the case of a negative antigen test in an asymptomatic person if there is a high probability of infection.
The diagnosis of infected people is crucial for the control and reduction of transmission. It usually marks the start of contact tracing.
Despite the great efficacy of RT-PCR as a diagnostic confirmation test, sometimes a negative result may not completely rule out SARS-CoV-2 infection.
If there is suspicion of infection, the results of the test should be evaluated in the clinical-epidemiological context of the patient and together with other diagnostic tests.
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