Remarkably Stable Virus-Like Positive Controls for COVID-19 Assays

Researchers have prepared a new class of positive control materials to validate the molecular assays that have been developed to detect and diagnose COVID-19 infection.

Nucleic acid tests, especially reverse transcription polymerase chain reaction (RT-PCR), have become the standards for early detection of COVID-19 infection. Positive controls for the molecular assays have been developed to validate each test and to provide high accuracy. However, most available positive controls require cold-chain distribution and cannot serve as full-process controls.


To overcome these shortcomings, investigators at the University of California, San Diego (USA) have developed new and improved positive controls. The investigators developed two different controls: one made from plant virus nanoparticles, the other from bacteriophage nanoparticles.

The plant virus-based controls were made using cowpea chlorotic mottle virus. The investigators removed the RNA contents of the virus and replaced them with a synthesized RNA template containing specific sequences from the SARS-CoV-2 virus.

Plasmids (segments of circular DNA) were used to make the bacteriophage-based controls. Plasmids containing gene sequences from the SARS-CoV-2 virus, as well as genes coding for surface proteins of the bacteriophage Qbeta, were loaded into bacteria. The bacteria then generated virus-like particles with SARS-CoV-2 RNA sequences on the inside and Qbeta bacteriophage proteins on the outside.

Results revealed that the positive control materials were detected accurately in COVID-19 molecular assays. The controls were stable and could be stored for a week at temperatures up to 40 degrees Celsius while retaining 70% of their activity after one month of storage.

"Our goal is to make an impact not necessarily in the hospital, where you have state-of-the-art facilities, but in low-resource, under served areas that may not have the sophisticated infrastructure or trained personnel," said senior author Dr. Nicole Steinmetz, professor of nanoengineering at the University of California, San Diego. "It is a relatively simple nanotechnology approach to make low-tech assays more accurate. "This could help break down some of the barriers to mass testing of under served populations in the United States and across the world."

The new COVID-19 positive controls were described in the November 25, 2020, online edition of the journal ACS Nano.

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