press release
Published: 01 December 2020

False positive Covid-19 tests may be the result of contamination in laboratories

By Natasha Meredith

A major cause of false positive Covid-19 test is contamination, as outlined in a new opinion piece in the journal Clinical Chemistry.

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In this unique article, initiated by the University of Surrey and the National Measurement Laboratory (NML) at LGC, scientists deliberated over false positive Covid-19 test results and why they occur. While initially false negatives may seem more of a concern, false positives (telling people they do have a disease when they don’t) can still cause problems within the healthcare system. For example people can be told to self-isolate unnecessarily, taking them away from their frontline duties and/or hospital in patients may be being placed on a Covid-19 ward unnecessarily.

Scientists in this piece specifically discuss the polymerase chain reaction (PCR), the principal diagnostic method used in the worldwide struggle against Covid-19, which can detect the nucleic acid from SARS-CoV-2 in a variety of patient samples. It is noted in the piece that when designed and used properly the PCR should not cross-react with similar but distinct viral genomes such as those which cause the common cold and should perform with very high analytical sensitivity.

It is highlighted by the team that although historical problems with PCR testing and contamination have existed, with reaction products contaminating samples resulting in false positives, robust measures and laboratory procedures can be put in place to prevent this happening. Despite such actions, concerns have risen within the scientific community about the rising number of false positives using this method to detect Covid-19.

Scientists believe that contamination may be happening within laboratories developing different components of (RT) - PCR tests. It is common practice when using PCR testing to make control materials synthetically. Yet each synthesis generates thousands of trillions of the very molecule of interest. As PCR performs with very high sensitivity, often able to measure single molecules, this represents a serious contamination risk. To avoid contamination, it is essential that the control template is made at different sites, usually from different vendors, from those making other PCR reagents used in the test.

Those involved in this piece conclude that due to the large numbers of laboratories developing assays and the same positive control materials for PCR testing, using different vendors may not be an option, increasing the risk of contamination. Such levels of production and subsequent contamination may not only generate false positives and reduce the sensitivity of the diagnostic method but it may also limit other areas of research.

To avoid false positives, scientists recommend a number of steps for laboratories to take to avoid contamination; including quality controlling reagents prior to their use, applying caution when results are close to the limit of detection of assay and taking preventive measures such as physically separating PCR set up and sample handling steps (and equipment) from those used for PCR analysis. It was also suggested that laboratories discard all reagents linked to contaminated reactions and deep clean their testing facility using proven solutions known to destroy nucleic acids.

Dr Jim Huggett, Senior Lecturer, Analytical Microbiology at the University of Surrey and Principal Scientist at the NML, said: “To tackle Covid-19, we need robust testing procedures in place to identify those who have the virus and stop its spread. However, false positives results undermine the confidence in diagnosis but are not difficult to identify if you know what to look for.  While there is tremendous pressures laboratories and test developers to deliver SARS-CoV-2 test solutions, it is important that in the rush to meet this need the steps required to ensure test quality are not overlooked. ”

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