Describing antigen testing for patients with COVID-19

In this video, we'll discuss the benefits and drawbacks of using antigen testing to detect COVID-19.

Franz Wiesbauer, MD MPH
Franz Wiesbauer, MD MPH
9th Jul 2020 • 6m read

Antigen testing offers a cheaper and faster method for diagnosing COVID-19 when compared to RT-PCR, but it comes with a significant limitation. In this video, from our COVID Mini: Diagnostics course, we'll look at how this test works and consider the benefits and drawbacks of integrating it into the COVID-19 diagnostic process.

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Video transcript

Antigen testing is a new diagnostic test for COVID 19 that received emergency use authorization by the Food and Drug Administration in May 2020. These new tests are not yet widely available, and their application in the COVID 19 pandemic is still rare. But they may offer a less expensive and more rapid detection of SARS cov to affordable and rapid testing may allow people to test themselves or COVID-19.

Rather than relying on getting tested at a screening center. People could potentially test themselves before going to work or school, visiting grandma. And using public transportation for example, people with a positive test can be isolated quickly based on local policies. With wider availability of these tests, we can better control the spread of COVID 19.

Think about how home antigen testing in combination with the tracing app could boost the effectiveness of contact tracing. in emergency rooms or doctor's offices. Medical personnel could use these tests to quickly triage patients and isolate those with positive results. So how does this antigen testing work?

SARS cuff to antigens, meaning viral proteins are detectable in upper respiratory tract secretions during the acute phase of infection. Similar to RT PCR testing antigens are sampled using nasal or nasal pharyngeal swabs. In the future, saliva samples might become more important. Also, collecting nasal pharyngeal swabs requires training and is not feasible at home.

Instead of testing for viral RNA, these tests detect pieces of the virus or antigens, such as the nucleo capsid protein that surrounds SARS cov. Two once a sample has been obtained, it will be processed with reagents to disrupt the particles of the virus and expose the viral nuclear proteins, which are the antigens that are detected by the test.

After a short time the process sample will be applied to the testing strip. Similar to lateral flow antibody tests. antigen testing uses a lateral flow immuno assay. The particles in the sample are transported across the testing strip by capillary effect, the particles first reach a section of the strip that tags the nucleo capsid protein with fluorescent labeled antibodies.

These labeled monoclonal antibodies bind to the nucleo capsid protein, the antibody antigen complex will continue to move further to the right on the strip until it reaches the test line. The test line is covered with immobile anti SARS cov two antibodies that will stop lateral movement and if the nucleo capsid protein is present, the test line will turn positive.

If no nucleocapsid protein is present in the sample, the line will not turn positive. Another line should appear next to the test line called the control line, which indicates the test is valid. The control line contains labeled antibodies that bind to non antigen targets in the sample like streptococcus see antigens that are normally found in the microbiome of the pharynx.

The antigen test is red as either positive with the test line and control line visible negative, with only a control line visible or invalid, with only a test line appearing or no line appearing at all. There are also newer devices that are digital and show positive, negative or invalid without the operator having to interpret the results of the strip's

RT PCR is the gold standard of diagnostic testing for COVID 19. But antigen testing has some advantages. One of the benefits of antigen testing is the high speed of testing compared with the standard RT PCR. Once the sample is added to the testing strip, it takes about 10 to 15 minutes for the results to appear. This time does not include the preparation of the sample prior to testing.

These procedures may differ from test kit to test kit. antigen testing is also less expensive compared to RT PCR. During the pandemic many of the raw materials needed for RT PCR are in shortage. Apart from RT PCR testing supplies personal protective equipment and personal training are all essential aspects of total cost and capacity for testing.

If people can one day to ask themselves in the comfort of their homes, they will not require personal protective equipment. In fact, antigen testing kits do not require extensive laboratory training, as most critical steps are automated in a single device. Although antigen testing sounds like a better alternative to standard testing, because it provides rapid results, the results are unfortunately not as sensitive.

Remember, sensitivity is the number of true positives divided by all diseased individuals. That's true positives plus false negatives times 100. And specificity is equal to true negatives divided by all non diseased individuals, meaning false positives, plus true negatives times 100. A test with a high sensitivity will have few false negatives, and the test with a high specificity will have few false positives.

Antigen tests do have a relatively high specificity, so there will be relatively few false positives. This means that when a patient tests positive, they are very likely infected with SARS cov to one of the limitations however, is the limited sensitivity, which means that we'll have a relatively high percentage of false negatives. Or in other words, the true positives will be low compared with PCR.

So this test will miss a lot of people that really have the disease. One study reported the sensitivity and specificity of an antigen test for SARS cov, two developed by a Belgian company, while the specificity for the test was high at 99.5%, the sensitivity was quite low, with only 57.6%. This means that among the disease population, the test can only detect 57.6% of the positive cases, a large percentage of disease patients may not be captured with the antigen test.

These findings of low sensitivity are consistent with that of influenza antigen tests. A study published in the Journal of Clinical microbiology reported a pooled sensitivity of 54.4% for influenza A antigen tests and 53.2% for influenza B antigen tests. This means that the influenza antigen tests may only detect a little more than 50% of positive influenza cases.

Due to the low sensitivity of an antigen test, a negative test results will not rule out COVID 19 infection because it may correspond to a false negative. People who do not have symptoms, were not in contact with an infected individual and did not travel to an endemic area have a low likelihood of being infected. So a negative test result reduces their likelihood of having COVID 19.

Even further. These people may feel confident when visiting family going to work or school and traveling with less fear that they're transmitting the virus to others. Patients with a high likelihood of being infected who test negative on an antigen test may need to be followed up with RT PCR, since the risk of false negatives is high.

If these tests are eventually designed for at home use, people will need to interpret negative results cautiously. antigen testing can be an extremely valuable diagnostic tool in a busy emergency room. Anyone who tests positive can be rapidly isolated and treated, and those who test negative may require additional follow up depending on the content of their recent exposure, history and the presence of clinical signs and symptoms consistent with COVID 19.

RT PCR is still the gold standard diagnostic tool for acute disease. However, new diagnostic tools may help alleviate costs of testing and enable more widespread testing. Interpreting RT PCR antigen and antibody tests together provides cross validation, improved sensitivities and informs the stage of COVID-19 patient's disease progression.

With improved sensitivities and easier sample collection antigen tests may become a valuable tool in diagnosing and controlling the spread of COVID 19.

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