Explained: How to test your smartphone screen for coronavirus

Written by Kabir Firaque, edited by Explained Desk | New Delhi |

June 25, 2021 6:09:51 PM

Scientists have developed a new method of testing for coronaviruses that cleans your smartphone screen rather than your nose or throat. Compared to regular PCR tests, the telephone screen test (PoST) method is non-invasive, costs less and is just as accurate, the scientists reported in the journal eLife.

Why smartphones

When people cough, sneeze, or speak, they expel droplets that settle on the surfaces around them. If a person is infected with SARS-CoV-2, these droplets will carry the virus. Previous studies have shown that SARS-CoV-2 can be detected from various types of surfaces, including their phones.

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“Smartphones are personal items that are constantly exposed to people’s mouths, their screens becoming a likely contaminated surface. Therefore, we hypothesized that individuals contagious Covid-19 would regularly deposit aerosols, droplets of saliva or upper respiratory tract secretions containing excreted SARS-CoV-2 virions, on their phone screen, which could then be sampled and detected by RT-PCR, ”write the authors.

The study was led by Dr Rodrigo Young of University College London. The UCL team conducted the study at Diagnosis Biotech, a Chilean start-up led by Dr Young.

How it’s made

PoST involves taking samples from the phone screen with regular swabs like those used for nasopharyngeal sampling, but incorporating them in a saline solution. “The sample is then subjected to regular PCR as with clinical samples,” Dr. Young said via email.

The study was performed on 540 double-blind people who underwent both regular PoST and PCR tests. The two tests were performed in different laboratories by independent teams, who were not aware of each other’s results.


The PoST detected the virus on the phones of 81.3% to 100% of contagious people with a high viral load.

Of the 540 people, 51 tested positive for SARS-CoV-2 by RT-PCR performed on nasal / throat swab samples. Of these samples, 15 had a low Ct value (less than 20) – and these also tested positive in PoST. This suggests that the ability of PoST to correctly identify positive individuals (sensitivity) is 100% in individuals with high viral loads, the authors said. For 29 other samples with mean Ct values ​​(less than 30), the sensitivity of the PoST was 89.7%.

The overall ability of PoST to correctly identify a negative case (specificity) was found to be 98.8%. Six samples were identified as positive in PoSt but negative in clinical swab testing. Although these could be interpreted as false positives, the study notes that two of these people had symptoms of Covid-19 and suggests that these two results could also be false negatives from regular PCR tests.

Why it matters

The authors presented PoST as an option for large-scale testing. They note that regular large-scale testing is difficult because accurate testing is either too invasive, expensive, or complicated to implement.

“The important thing to take into account is that many infected and contagious people are not symptomatic. Therefore, they unknowingly spread the virus, ”Dr. Young told The Indian Express. “… If we can manage to perform periodic mass testing (on) all people without symptoms, we could theoretically end the pandemic. “

A machine is currently under development by Diagnosis Biotech, Dr. Young’s start-up. According to the UCL website, the machine will build on this research, taking a phone for PoST sampling and providing the results directly via SMS to minimize contact.

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