An antigen-based detection technique developed by University of Helsinki researchers could be used to analyze as many as 500 samples per hour.

In a recent study, the test was able to diagnose a viral infection almost as accurately as PCR tests, which are known for their sensitivity, describes Elina Kirvesniemi/University of Helsinki .

Time-Resolved Förster Resonance Energy Transfer

The test is based on a phenomenon known as time-resolved Förster resonance energy transfer (TR-FRET), where energy travels between two light-sensitive molecules when they are close enough to each other. TR-FRET makes it possible to measure viral particles or the body’s own proteins by using what are known as ‘mix and read’-type tests on complex biological samples, such as serum or even whole blood.

In practice, the TR-FRET solution of the new SARS-CoV-2 rapid test functions like this: a nasopharyngeal swab taken from the test subject is mixed in a test solution which contains antibodies that recognize the SARS-CoV-2 nucleoprotein or spike protein. The antibodies marked with fluorescent labels bind with SARS-CoV-2 particles, forming molecular assemblies, or complexes, whose existence can be confirmed/detected by using a TR-FRET assay. The results come in roughly 10 minutes later: the formation of any complexes demonstrates, to a high degree of certainty, an infection caused by SARS-CoV-2 in the test subject.

“We demonstrated in our study that a technique based on the TR-FRET phenomenon can be used to diagnose SARS-CoV-2 infections in clinical specimens,” says Jussi Hepojoki, docent of virology and Academy of Finland research fellow at the University of Helsinki.

“We demonstrated that the technique we have developed was able to detect almost all positive specimens (37/38), from which we were able to isolate SARS-CoV-2 in cell culture.”

The researchers investigated the functioning of the rapid test using 48 specimens which had been selected on the basis of a positive SARS-CoV2 PCR test, with varying concentrations of viral RNA. “We demonstrated that the technique we have developed was able to detect almost all positive specimens (37/38), from which we were able to isolate SARS-CoV-2 in cell culture. In other words, the carriers were likely to continue to spread the virus at the time of sample collection,” says Hepojoki.

In contrast, 10 of the selected group of positive SARS-CoV-2 samples produced a positive result in a PCR test even though virus isolation was no longer possible. None of these samples yielded a positive antigen test result.

Carry out testing almost anywhere

A TR-FRET reader roughly the size of a desktop computer is needed for the test, making it possible, at least in theory, to carry out testing almost anywhere.

“The theoretical capacity of the test is very high. According to our calculations, it would be possible to manually analyse as many as 500 samples per hour, with one person doing the testing and using a single testing device. Also, the cost of test reagents is fairly low,” Hepojoki notes.

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