October 16, 2021

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Do droplets and airborne contamination cause false positives in COVID-19 tests?

The rapid spread of the coronavirus pandemic in 2019 has called for immense testing capabilities as a cornerstone in the fight against infections. There are, of course, concerns about false positives due to the high number of people testing positive for SARS-CoV2. A new article from the University Hospital Zurich and the University of Zurich in Antimicrobial resistance and infection control investigated whether the sample tubes were inadvertently contaminated with infected individuals.

Global spread and measures

Since the end of 2019, tSevere Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has spread around the world, causing more than 119,000,000 confirmed cases and more than 2,600,000 deaths.

A cornerstone of infection control and prevention aimed at the early detection of incident cases requiring immense testing capacity.

Multiple public health measures have been implemented to slow the spread of COVID-19, including social distancing, reduced contact and mobility, isolating sick people and quarantining those exposed. A cornerstone of infection control and prevention aimed at the early detection of incident cases requiring immense testing capabilities.

SARS-CoV-2 test capabilities

The indications for SARS-CoV-2 testing evolved over time in light of the availability of testing capabilities. Initially, capacity was limited and testing was given priority in patients requiring hospitalization due to the severity of the disease and / or in those at increased risk of serious disease. With greater availability of SARS-CoV-2 tests, the barrier to rapid tests has been lowered, resulting in more than 60,000 tests / 100,000 inhabitants from January 24, 2020 to March 15, 2021 in Switzerland and Liechtenstein. This immense increase represented enormous efforts for the public health system. To meet these demands, designated SARS-CoV-2 testing centers have been set up. These centers focus on people in need of SARS-CoV-2 testing, but who have only mild illness or are asymptomatic, and this strategy was also aimed at maintaining the capacity of health systems.

With this in mind, SARS-CoV-2 testing centers can be seen as a place for people with SARS-CoV-2 positive. Notably, around 11.5% of all PCR-based SARS-CoV-2 tests in Switzerland and Liechtenstein were positive between January 24, 2020 and March 15, 2021.

Given the high rate of SARS-CoV-2 positivity among those tested at the testing centers, concerns about contamination resulting in false positive results for SARS-CoV-2 have been raised.

The question of false positive SARS-CoV-2 tests

In our recent article, we investigated the possible contamination by droplets or particles suspended in the air, propagated by the tested individual, in a designated SARS-CoV-2 test center in Switzerland. 40 unsealed sample containers were placed at different distances from the test individual (up to 1 meter) in differently ventilated rooms for 180 min. All the samples remained negative, despite the fact that more than 18% of the individuals tested positive during the period of investigation.

Many studies have looked at the transmission of SARS-CoV-2 and have identified droplet transmission as by far the most relevant, while aerosol and contact transmission probably contribute to a lesser extent (follow these links to learn more more about the Center for Disease Control and Prevention and the World Health Organization) In experimental contexts, viral RNA has remained detectable on different surface materials for several hours. Given the high rate of SARS-CoV-2 positivity among those tested at the testing centers, concerns about contamination resulting in false positive results for SARS-CoV-2 have been raised. In addition, possible false positive or false negative results due to analytical procedures, pre-analyzes should also be taken into account.


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