COVID-19 antigen test could give quick results, but is not foolproof

The cheap, simple-to-use test could miss as many as 4 in 10 cases of SARS-CoV-2 infection. However, its developers argue that as part of triage for suspected cases, the test could help conserve scarce resources for more sensitive laboratory testing.

At the time of writing, SARS-CoV-2 — the coronavirus that causes COVID-19 — has infected 4.2 million people worldwide and caused more than 291,000 deaths.

One of the main weapons being used to curb the spread of the virus is testing for infection.

But the capacity of healthcare systems to carry out the most sensitive, reliable laboratory test for the virus is limited, and there have been worldwide problems with the supply of reagents. In addition, it can take 1–2 days for results of a lab test to come back.

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Easy-to-use point-of-care tests, which give a result in under 30 minutes without the need to send the sample to a lab, offer a cheap alternative.

The tests are based on a monoclonal antibody that latches onto a viral antigen, which is a characteristic part of a protein on the surface of the virus.

Compared with lab-based testing, point-of-care tests are less sensitive. They are more likely to miss cases of the infection, giving a result known as a false negative.

Nonetheless, as part of a triage system, point-of-care tests could take some of the pressure off hospital laboratories at the peak of an outbreak by rapidly screening large numbers of people who may have symptoms consistent with a SARS-CoV-2 infection.

In advice issued on April 8, 2020, the World Health Organization (WHO) backed the use of such tests in principle, providing that their effectiveness can be established:

“If any of the antigen detection tests that are under development or commercialized demonstrate adequate performance, they could potentially be used as triage tests to rapidly identify patients who are very likely to have COVID-19, reducing or eliminating the need for expensive molecular [lab-based] confirmatory testing.”

New antigen test

A Belgian biotech company called Coris BioConcept has created a new point-of-care test by repurposing a test originally developed to detect the closely related coronavirus SARS-CoV, which causes the respiratory infection SARS.

Called “COVID-19 Ag Respi-Strip,” the test is only commercially available to healthcare services. It is not for sale to the general public.

Like a home pregnancy test, it is a “lateral flow test,” comprising a reagent strip enclosed in plastic casing. If the test is positive, a colored band appears at a marked test line.

Researchers gauged the sensitivity and specificity of the new test against the gold-standard laboratory test for SARS-CoV-2, which uses the highly sensitive polymerase chain reaction to detect genetic material of the virus.

In a study paper published in Frontiers in Medicine, the researchers explain that they tested 328 nasopharyngeal swab samples from people with symptoms of SARS-CoV-2 infection.

Their results showed an overall sensitivity of 57.6%, which means that, on average, the test will identify around 6 out of every 10 people who have the infection.

Put another way, around 4 in 10 people with the infection would receive a negative test result.

However, among the subset of samples containing a high viral load — taken from the patients with the most contagious infections — this test will detect 7 out of 10 cases.

The test had an overall “specificity” of 99.5%, meaning that almost all of the samples that tested positive were from patients who had the infection. In other words, there were very few false positives.

Importantly, when the researchers tested samples containing other pathogens, including those that cause pneumonia, the common cold, and influenza, none tested positive.

As expected, samples containing SARS-CoV tested positive, but those that contained other coronaviruses tested negative.

Immediate care

The researchers conclude that, in a hospital or primary care setting, patients who test positive could be quarantined and receive immediate care, while those who test negative would need to await the results of a standard lab test.

In their paper, the team proposes that the test might help contain outbreaks in countries where healthcare resources are very limited. They write:

“A major usefulness of the COVID-19 Ag Respi-Strip test would be in the low- and middle-income countries, where molecular assays are available in very few laboratories, mainly only in capital cities. The detection of viral infections in patients attending primary care centers would allow healthcare workers to rapidly identify new outbreak foci and define quarantine measures for high viral shedders and/or suspect patients to limit the spread of the epidemic.”

Still, the scientists emphasize that the test only complements laboratory testing — it is not sensitive enough to replace it.

“The sensitivity of the COVID-19 Ag Respi-Strip is too low to meet the requirements of a frontline standalone triage test, and this limitation must be clearly communicated,” they write.

Independent trials

Several of the study’s authors work for the company that developed the test. In addition, the trials were performed in a laboratory on stored samples from hospital labs, rather than in a clinical care setting.

Larger, independent trials of the test in clinical settings would be needed before the test could be widely adopted by healthcare providers.

Senior author Dr. Olivier Vandenberg, a professor at the Université Libre de Bruxelles, in Belgium, told Medical News Today: “We are now assessing the clinical impact of this test. We are also trying to identify the population [for whom it] would be the most beneficial.”

In its advice about point-of-care tests, the WHO applaud the efforts of test developers to innovate, but say that data concerning the tests’ effectiveness are currently insufficient to recommend their use among the wider population:

“Before these tests can be recommended, they must be validated in the appropriate populations and settings. […] At present, based on current evidence, WHO recommends the use of these new point-of-care immunodiagnostic tests only in research settings. They should not be used in any other setting, including for clinical decision-making, until evidence supporting use for specific indications is available.”

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