A study of the university of Navarra concludes that rapid tests for the detection of SARS-CoV-2 (known as antigen test) are less effective when performed in asymptomatic population and, therefore, they are not useful tests for mass screening.
The study, directed by the doctor in charge of the Covid Area of the University of Navarra, Alejandro Fernandez Montero, and published in the magazine ClinicalMedicine, highlights that the rapid antigen test it is a useful tool when performed in a population with symptoms and it is interpreted appropriately, but in low-risk populations the proportion of false negatives is higher and therefore a negative result does not rule out Covid infection.
When carried out in symptomatic patients, the antigen test has been shown to have high sensitivity and specificity, meeting the criteria of the World Health Organization (WHO). The study has also found that the sensitivity of the antigen test (the ability to detect infected individuals) is directly related to the viral load in the body. The test is capable of detecting high viral loads, but not detecting lower viral loads, which may correspond to the onset or end of coronavirus infection.
The study data
2,543 people, including employees and students of the University of Navarra, participated in the study that was carried out between the months of November 2020 and January 2021. The objective was to validate the rapid Covid19 antigen tests by comparing them with the PCR tests, carried out within the screening program that was carried out in the academic center. The only exclusion criterion was the presence of symptoms compatible with Covid-19 (fever, persistent cough, or shortness of breath). People with nonspecific symptoms or close contacts were not excluded. In fact, the researchers divided the population into low-risk participants (asymptomatic) and higher-risk participants (contacts or mild symptoms) by analyzing how the rapid test performed in each group.
Two samples were taken from each participant and the two tests, PCR and antigen test, were performed. The results of the PCR tests yielded a percentage of positivity of 1.93%: 49 positive cases, of which 35 were also detected by the rapid antigen test and 14 were not. This implies that almost 29% of the positives were not identified by the rapid test (sensitivity equal to 71.43%). This percentage of false negatives increased further, up to 47%, in the case of low-risk and asymptomatic participants. The sensitivity of the test decreased to 53.33%.
“The problem is that in these positive cases that remain undiagnosed by the antigen test, we do not know if they are cases in which they are already eliminating the infection and therefore not contagious, or the person is at the beginning of the disease and in a short period of time it will have a very high contagion capacity, “says Dr. Alejandro Fernández-Montero.
The results of the study reinforce current WHO recommendations that the validity and sensitivity of these rapid tests should be better evaluated and therefore they are not currently recommended for the diagnosis of coronavirus infection in asymptomatic populations. Researchers advocate improving antigen test screens by using clinical questionnaires to select the highest-risk participants, confirming negative results with PCR, or using repetitive sequential rapid tests.
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