Rapid antigen tests, which are commonly used in the diagnosis of respiratory pathogens, detect the presence of a specific viral antigen, implying a current viral infection.
Detecting the presence of the SARS-CoV2 virus (COVID-19) in a person’s body, therefore, allows the infection to be diagnosed on the spot, so that measures can be taken to prevent further transmission.
The optimal moment for the detection of Antigens is similar to that of RT-PCR, that is when their viral load (number of viruses) reaches the maximum concentration in the samples used (nasopharyngeal, oropharyngeal, salivary, bronchoalveolar).
For symptomatic patients, it would be from a few days before the onset of symptoms (2-3) and during the first 5-7 of their evolution.
For asymptomatic patients, viral load is most easily detectable from about 4-5 days after the onset of infection and for 7-10 days.
These times are approximate, they vary between different individuals and according to the quality/processing of the sample, as can be seen in published studies .
The information contained in this response is under continuous review based on updates from ongoing clinical studies and scientific evidence.
It is currently known that antigenic tests are especially accurate in symptomatic individuals, especially in the five days after the onset of symptoms, as well as in asymptomatic close contacts, in the first seven days after exposure to a confirmed case of COVID- 19. The margin of error with symptomatic people is low.
The latest published studies affirm that they are also effective in asymptomatic people, although the sensitivity rate is lower.
Rapid antigen tests are especially indicated for the rapid and efficient detection of patients infected by COVID-19 who live in environments where there is a high risk of community transmission (work centers, residences, schools, hospitals, etc. hotels, prisons, massive cultural and sports activities, etc).
Antigen tests are less accurate than PCR, but faster: 15 minutes instead of a couple of days, which could help control large disease outbreaks.
The saliva test in the studies provided by the manufacturer, provides a sensitivity of 97.3% and a specificity of 99.6% in a study on 500 patients.
This allows rapid antigen tests to be used for massive and preventive screening for the early detection of infected people and to guarantee safe collective environments.
In the interference study, the impact of possible components (such as different medicines or foods) that could interfere with the accuracy of the test results was evaluated, demonstrating a total absence of interferences in the saliva samples.
In the nasopharyngeal samples, the results showed that they did not have any interference with endogenous or exogenous substances, except a very weak one with HAMS Type I.
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