Pregnancy-style tests to detect SARS-CoV-2
The diagnosis of SARS-CoV-2 infection in asymptomatic carriers is crucial to help limit the spread of the virus and prevent new strains from emerging; to do so, it was important that the method used for mass testing could be carried out easily and provide rapid results. And so led to the introduction of lateral flow testing which operates on a similar basis of enzyme-linked immunosorbent assays (ELISA) like pregnancy tests.
Both are highly specific in detecting antigens and hormones and does not require a large sample volume to be able to work. With pregnancy tests, the test strip is bound by antibodies that bind the hormone, human chronic gonadrotropin, that is produced in high levels when a woman is pregnant; the hormone acts as an antigen and upon binding this will cause a colour change. This indicates a positive result because a new line formed which is separate from the control line that indicates that the sample is flowing in the right direction. Alternatively, if the hormone is not present, the antibodies having nothing to bind to and all that can be read is the control line — the result is negative.
In comparison, when testing for the SARS-CoV-2, antibodies are designed to bind to the spike glycoprotein on the surface of the virus, this is an important target because this protein interacts with a receptor (ACE2) that is abundant on many human cells to gain entry and infection. The reason why lateral flow is used is because this is much easier to perform for rapid results but the sensitivity varies which is why it has been developed to work similarly to ELISA. This has been very successful across England as people can find out whether they have been infected by the virus within 30 minutes, compared to PCR (polymerase chain reaction) which can take hours to days. However, recent reports from the mass testing carried out in Liverpool stated that there is much more work to be done to improve the reactivity as statistics showed that 3 in 10 cases of positive participants were missed.
The difference in accuracy could be down to the fact that lateral flow is based on detecting the antigen-antibody interaction as opposed to PCR that focuses on detecting the viral genetic material. rt-PCR (reverse transcription polymerase chain reaction) is the variation used because SARS-Cov-2 carries RNA that needs to be converted to DNA, which is then amplified to detectable levels to determine whether or not the virus is present in the individual. A comparative study was carried out in the past with the human Metapneumovirus which had results consistent with the Liverpool mass testing in that rt-PCR showed a much higher degree of specificity and sensitivity than lateral flow; however, the study also supported the use of lateral flow as a rapid and useful means for diagnosis of viral infections. Nonetheless, the proportion of false negative read outs is alarming, there is much room from improvement to devise a strategy that confers rapid testing with high accuracy in order to help break the chain of spread from asymptomatic carriers.
