What should the Covid-19 test look like?

November 28, 2021 First Edition
Updated 18 hours ago

Idea room

As the pandemic competes for the enviable two-year milestone Otago Global Health InstituteThe Covid-19 Master Class series brings together a network of professionals to discuss key topics in Covid-19. I will produce works every day until December 5th.

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James Ussher and Philip Hill will evaluate current testing options and those available around the world and offer some suggestions on what to do.

Diagnostic testing is essential for covid-19 compliance, but sufficient testing power and speed to obtain results is a major global challenge.

People with Covid-19 symptoms should be tested as soon as possible. If they seek testing and the results are readily available, the public health team will have the opportunity to find their contact information and stop further transmission of SARS-CoV-2.

Covid-19 is highly infectious for up to two days before the infected person becomes symptomatic and requires surveillance testing of asymptomatic individuals in certain situations.

As the pandemic continues, many companies and organizations will develop testing strategies. These should be developed in collaboration with health professionals, considering the following issues:

How do you estimate the performance of your Covid-19 test?

The test properties indicate how to use the test. Estimate their sensitivity (percentage of people who are positive for the virus) and specificity (percentage of people who are virus-free and negative). If the test is less specific, there will be too many false positives.

It also estimates the predicted value. A positive predictor indicates what percentage of people who test positive are infected with the virus. Negative predictions indicate what percentage of people who test negative are not infected with the virus.

Predictions vary depending on the prevalence of infection in the population. For example, if the test is 80% sensitive and 98% specific (such as the current rapid antigen test described below), and the community prevalence of Covid-19 is 1%, the predicted positive value is 30%. Is less than.

SARS-CoV-2 often detects both viable and infeasible viruses in the test, complicating the interpretation of the results.

What kind of test was developed?

Laboratory-required Covid-19 tests tend to use high-throughput polymerase chain reaction (PCR) machines, with high sensitivity (90% and above) and very high specificity (nearly 100%). increase. The two main sampling methods for these tests are nasopharyngeal swabs and saliva samples.

The sensitivity of the highest saliva test has reached the same level as the nasopharyngeal test. However, laboratory transport, multiple sample processing, batch testing, and patient reporting often require more than 24 hours to wait for results.

Some machines run PCR tests after a sample is placed in a cartridge and the results are available within an hour, but only a few cartridges can be processed at one time and cartridge availability is severely limited worldwide. ..

A further simplification of these methods for detecting viral RNA is loop-mediated isothermal amplification (LAMP), which does not require a laboratory and provides results in less than 30 minutes. So far, few studies have evaluated the performance of the LAMP test and it seems to be less sensitive than the PCR test.

Covid-19 Rapid Antigen Test (RAT) is becoming more and more available. RAT detects the presence of viral “peplomers”. They tend to use nasal swab samples, and so far few large studies have evaluated saliva as a sample.

RATs differ significantly in terms of performance characteristics and turnaround time, and can typically only process one sample at a time. There is a difference in sensitivity between symptomatic (about 65% on average) and asymptomatic (about 55% on average) individuals.

Reported sensitivity estimates are often PCR test. Sensitivity also varies at different stages of the disease, with an average of about 70% in the first week and about 45% in the second week. Often, the specificity is very high (close to 100%) and there are many of higher quality antigen tests.

Some of the more sensitive (close to 90%) have a slight decrease in specificity up to 97%. If the infection rate is low, this can result in the majority of positive tests being false positives.

Other diagnostic tests under development use microfluidics, holographic microscopy to examine nanoscale structures and use techniques such as gas chromatography. Breath tests and trained detection dogs have some potential.

Wastewater sampling PCR-based testing is hampered by fluctuations in the number of cases that release the virus in the faeces, dilution of the virus, and the inability to distinguish between viable and non-viable viruses.

What testing strategy should I use?

While many countries have struggled to establish test capacity large enough to effectively respond to outbreaks, the lack of strategic use of large test capacity is a serious inefficiency in other countries and thereafter. It led to the failure to contain the outbreak.

Symptomatic cases are much more infectious than those who never become symptomatic, and testing for asymptomatic people can lead to a shift in testing from symptomatic individuals.

In addition, asymptomatic people probably play a small role in overall communication, so numerous positive results from such individuals may put unnecessary work on the health response.

When it comes to testing, you can think of at least five different groups of people.

1. Asymptomatic people in the community should be able to take the test. To maximize uptake, both nasopharyngeal swabs and salivary PCR tests should be available.
2. Everyone entering the country must be subject to a test plan coordinated based on expert advice, combining pre-departure, arrival and post-arrival tests.
3. Symptomatic and asymptomatic individuals who are admitted to a risky situation (such as a hospital) should be included in a test plan to protect vulnerable people. Screening for all visitors and emergency department participants may not be feasible.
4. Symptomatic and asymptomatic individuals who participate in large-scale events that pose a superspreader risk are candidates for self-testing with RAT prior to participation, and requirements may be affected by vaccination status.
5. Occurrences differ in relation to the strategic use of the test. To maximize the value of limited laboratory resources, you can avoid testing for asymptomatic accidental contact.

* Statement of Conflict of Interest: Associate Professor James Asher is employed by the Southern Community Laboratories in Dunedin and is conducting a Covid-19 test. Professor Phillip Hill participates in four COVID-19 Ministerial Advisory Boards.