A study from the University of Navarra concludes that rapid tests for the detection of SARS-CoV-2 (known as antigen tests) are less effective when performed in an asymptomatic population and, therefore, are not useful tests for mass screening.

The study, led by Dr. manager of area Covid of the University of Navarra, Alejandro Fernández Montero, and published in the journal EClinicalMedicine, highlights that the rapid antigen test is a usefultool when performed in population with symptoms and interpreted properly, but in low-risk populations the proportion of false negatives is higher and therefore a negative result does not rule out Covid infection. 

When performed in symptomatic patients, the test antigen test has been shown to have a high sensitivity and specificity, meeting the criteria of the World Health Organization (WHO). The study also found that the sensitivity of the test 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 of detecting lower viral loads, which may correspond to the beginning or the end of coronavirus infection.

The data of the study

2,543 people, including employees and students of the University of Navarra, participated in the study, which took place between November 2020 and January 2021. The goal was to validate the Covid19 rapid antigen tests by comparing them with the PCR tests, performed as part of the screening program carried out at the academic center. The only exclusion criterion was the presence of symptoms compatible with Covid-19 (fever, persistent cough or shortness of breath). Individuals with nonspecific symptoms or close contacts were not excluded. In fact, the investigators divided the population into low-risk participants (asymptomatic) and higher-risk participants (contacts or mild symptoms) by analyzing how the rapid test behaved in each group.

Two samples were taken from each participant and both PCR and antigen tests were performed. The results of the PCR tests showed a positivity rate 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 even more, 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 are not contagious, or if the person is at the beginning of the disease and in a short period of time will have a very high capacity for contagion," says Dr. Alejandro Fernández-Montero.

The study results reinforce current WHO recommendations that the validity and sensitivity of these rapid tests need to be better evaluated and therefore are not currently recommended for the diagnosis of coronavirus infection in asymptomatic populations. The investigators advocate improving antigen test screening by using clinical questionnaires to select participants at higher risk, confirming negative results with PCR, or using sequential repeat rapid tests.