Lateral flow device (LFD): A mass testing technique for COVID-19

Ananya Dutta, Bose Institute

Various national and international health organizations have emphasized the significance of testing each and every individual in a population and ensuing the contact tracing in order to arrest the outspread of the COVID-19 virus. The existing diagnostic approach is a two-step procedure involving a reverse transcriptase-polymerase chain reaction followed by real-time PCR (RT-PCR) of the swabs collected from the nose/throat. This procedure is conducted at dedicated labs and the results are issued slowly (24-72 hrs), whereas the two-step method requires a few hours. However, the significant challenge is creating a testing facility for a large population of infected and asymptomatic individuals. So the need of the hour is a diagnostic approach that is quick, affordable, and precise with the capability to identify infected individuals.

What are LFDs?

Healthcare setups routinely use Lateral flow device (LFD) immunoassays to test diverse protein targets. This assay is affordable, easy to use and detection time is short with defined high accuracy.  In a few words, there is a conjugation pad containing bound antibodies, the sample is loaded onto it and the analyte (antigen) forms a complex with the antibodies. The analyte-antibody mix then migrates to a membrane via capillary flow reaction across “test” and “control” strips. The strips are layered with antibodies to detect the analyte. The appearance of colored control and test lines confirms the test as positive. Newly designed SARS-CoV-2 antigen LFDs are coated with antibodies that can identify spike,
envelope, membrane, or nucleocapsid proteins of the virus. LFDs are simple to use and so there is no requirement for trained personnel and highly advanced equipment making it an attractive option in mass testing. Moreover, In comparison to the RT-PCR procedure, the detection time is 10-30 mins in LFDs.

The UK’s Department of Health and Social Care (DHSC) from PHE Proton Down and the University of Oxford conducted an evaluation of the first generation of SARS-CoV-2 antigen-detecting LFD for rapid point-of-care (POC) testing. The LFDs were evaluated in 4 different phases. 

The evaluation concluded that LFDs can be a testing technique for COVID-19 but, there are certain shortcomings in the development of these kits and handling techniques. The takeaway message of the evaluation of the LFDs was to focus on the future research of achieving diagnostic precision and approaches to advance the sensitivity and correctness of these tests. These accomplishments will benefit customary serial sampling, digitally interpreting the results, and incorporating training policies utilizing videos and thus improving accuracy and reduction of transmission. 

Figure 1: Lateral Flow Device: The image depicts a common Lateral Flow Device. The “S” denotes the combination pad where the sample is loaded. The red lines on the strip portion depict a positive result. “C” and “T” are control and test respectively.

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Source: COVID-19: Rapid antigen detection for SARS-CoV-2 by lateral flow assay:
A national systematic evaluation of sensitivity and specificity for mass testing. Peto T. EClinicalMedicine 000 (2021) 100924. https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(21)00204-2/fulltext

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