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22Jun

Feature Piece - What You Need to Know About COVID-19 Testing

The below graph provides a view of English daily lab-confirmed COVID-19 cases, showing a marked decrease from the point of lockdown restrictions being introduced.

 

As the Government strategy moves to the Test and Trace Programme, a capacity for large-scale testing is critical. Quick communication from the centralised platform is also important. Testing on an extraordinary scale and effective tracing will ensure that the virus is kept below an ‘R’ number of 1.

In terms of diagnostic test development and validation, stages to be passed are:

  1. Analytical validation – includes questions to be answered such as are the materials and reagents consistent and reliable from batch to batch, and from individual device to device? Do different users get the same results with same samples?
  2. Clinical validation – this relates to sensitivity and specificity. Sensitivity is the proportion of all true positive samples called positive by the test. Specificity is the proportion of all true negative samples called negative by the test. It is key to understand that these values are comparatives not absolutes. An example of misleading sensitivity and specificity could be the evolving nature of antibody levels which may generate varying or incorrect diagnoses of active disease.
  3. Clinical utility – this concerns an evaluation of the in vitro diagnostic tool’s clinical effectiveness. Although a nasopharyngeal swab test may detect the illness in a person, does this mean they will develop the disease if symptoms are absent when tested? Testing methods for different disease will necessitate different priorities in terms of specificity and sensitivity ratios. For example, COVID-19 testing needs a high sensitivity level (a slight compromise on specificity is acceptable) and HIV testing demands 100% sensitivity.

Perceived reasons for COVID-19 include:

  • To confirm a diagnosis of COVID-19, by a treating physician on an individual level or by the public health system for isolation procedures.
  • To establish an individual is not infectious, usually for communal reassurance in group settings.
  • To understand the epidemiology of COVID-19 and its prevalence within a community.
  • To understand immunity caused by COVID-19.

No single test can address all these needs. Focussing on single-test number targets distract from the need for varied and effective use of a mix of testing methods to fulfil as many of these needs as possible.

Viral material detection tests fall into two basic categories: those to detect the virus itself (either viral RNA or viral protein); and those to detect an immune response to the virus.

  1. Reverse Transcription Polymerase Chain Reaction (RT-PCR) methodologies are used to detect SARS-CoV-2 viral RNA. Many of these tests distinguish this RNA from that of other viruses such as SARS, MERS. As of 7 June 2020, there were at least 146 commercially available in vitro diagnostic (IVD) PCR kits. All but 8 of this number claim some regulatory approval (either CE mark for Europe or EUA in the USA). Many of these tests will have been granted temporary approval through self-certification of their own product by authorities. Although RT-PCR is a relatively quick technology (running the test manually from sample to result can take 3-4 hours, and automated systems can reduce this time) delays can occur due to logistical issues with sample transport, results review, and return of the results. Although PCR testing has excellent specificity and sensitivity, questions remain over infectivity as individuals may harbour the virus for days before showing symptoms. PCR tests will only confirm a current positive case.
  2. The need for a more rapid turnaround time of results and perhaps for greater sensitivity in samples such as saliva has driven the invention and development of alternatives to classical RT-PCR. Some EUA-approved tests use isothermal amplification technology, a very quick detection process. CRISPR methods use enzymatic detection methods to signal the presence of nucleic acid within one hour. Other systems such as a multi-panel PCR testing approach that tests several samples within an hour have yet to receive full validation but are bound to become more popular.
  3. A device and reagents that qualitatively detect the presence of the nucleocapsid protein is a new alternative to nucleic acid testing. Results are produced in minutes by trained staff and the sample can be taken at the point-of-care before processing.

Antibody testing is the other category of IVD testing for COVID-19. The time course of the antibody response to COVID-19 is now being better understood and the risks of cross-reactivity with antibodies to other coronaviruses. Enzyme-linked immunosorbent assays (ELISA) and immune fluorescence assays (IFA) have been developed using recombinant S (S1 and S2) (13) and N proteins as antigen to capture and detect antibodies. As of 7 June 2020, there are 38 antibody IVD kits with some sort of regulatory approval (up from 28 a month previously). MHRA has released specifications for antibody self-tests and point-of-care tests. For use in mass strategy tests, a minimum 98% sensitivity and 98% sensitivity will be required.

The piece concludes that importance of IVD has been made clear to a wide audience due to the COVID-19 pandemic. If there is any advantage from the response to the virus, it would be the increased recognition of the skills and talent of those who invent, manufacture, evaluate and deploy diagnostics in the UK and further afield.

 

Journal Title: Faculty of Pharmaceutical Medicine

Authors: Robert Holland FFPM and John Bagshaw

URL Access: https://www.fpm.org.uk/blog/what-you-need-to-know-about-covid-19-testing/

 

References

  1. https://www.360dx.com/coronavirus-test-tracker-launched-covid-19-tests
  2. https://sherlock.bio/sherlock-biosciences-receives-fda-emergency-use-authorization-for-crispr-sars-cov-2-rapid-diagnostic/
  3. Laurenco J et al https://www.medrxiv.org/content/10.1101/2020.03.24.20042291v1

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