Antigen tests for COVID-19

7th Jan 2021

Antigen testing is a new diagnostic test for COVID-19 that received emergency use authorization by the Food and Drug Administration (FDA) 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-2 than current testing modalities such as reverse transcription polymerase chain reaction (RT-PCR).

Affordable and rapid testing may allow people to test themselves for 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. 

In emergency rooms or doctor’s offices, medical personnel could use these tests to quickly triage patients and isolate those with positive results. People with a positive antigen test can be isolated quickly based on local policies. With wider availability of these tests, we can better control the spread of COVID-19. Just think about how home antigen testing in combination with a tracing app could boost the effectiveness of contact tracing!

 

So, how does antigen testing work?

SARS-CoV-2 antigens—namely, viral proteins—are detectable in upper respiratory tract secretions during the acute phase of infection. Currently, similar to RT-PCR testing, antigens are sampled using nasal or nasopharyngeal swabs. Collecting nasopharyngeal samples requires training and is not feasible at home. 

Instead of testing for viral ribonucleic acid (RNA), antigen tests detect pieces of the virus, or antigens, such as the nucleocapsid protein that surrounds SARS-CoV-2. Once a sample has been obtained, it will be processed with reagents to disrupt the particles of the virus and expose the viral nucleoproteins, which are the antigens that are detected by the test. 

Three steps of SARS-CoV-2 viral protein collection and isolation: Swab in nasal cavity, swab in test tube, viral particles in test tube. Illustration.

Figure 1. Collection and isolation of the SARS-CoV-2 viral proteins that are used for the antigen test, a) collection via nasopharyngeal swab, b) processing of the sample with reagents, c) exposure of viral proteins.
 

Similar to antibody blood testing, antigen testing uses a lateral flow immunoassay (LFIA). The particles in the sample are applied to the left side of a test strip and transported across the strip by capillary effect. The particles first reach a section of the strip, the conjugate pad, that tags the nucleocapsid protein with fluorescence-labeled antibodies. These labeled monoclonal antibodies bind to the nucleocapsid 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-2 antibodies that will stop lateral movement, and, if the nucleocapsid 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, called the control line, should appear next to the test line which indicates the test is valid. The control line contains labeled antibodies that bind to non-antigen targets in the sample like streptococcus C antigens that are normally found in the microbiome of the pharynx. 

Steps of lateral flow immunoassay (LFIA) for detection of SARS-CoV-2: sample applied to test strip, viral proteins bind to antibodies, antigen-antibody complex shows positive result at test line, non-antigen targets bind to antibodies at control line. Illustration.

Figure 2. The lateral flow immunoassay (LFIA) for detection of SARS-CoV-2, a) prepared sample is applied to the test strip, b) particles move across test strip and viral nucleocapsid proteins bind to antibodies in the conjugate pad, c) anti-SARS-CoV-2 antibodies stop the movement of the antigen-antibody complex at the test line and show a positive result if nucleocapsid proteins are present, d) non-antigen targets bind to antibodies in the control line indicating the test is valid.​

The antigen test is read as either positive (with a test line and control line visible), negative (only control line visible), or invalid (only test line appears or no line appears). Newer devices have digital immunoassays that display the result as positive, negative, or invalid, rather than relying on the operator to read the results off the strip.

 

Positive, negative and invalid test results of a lateral flow immunoassay (LFIA) antigen test for SARS-CoV-2. Illustration.

 

Figure 3. Completed lateral flow immunoassay (LFIA) antigen tests for SARS-CoV-2 are read as positive, negative, or invalid based on the number of visible lines that appear on the test strip. A positive test will show a visible test line and control line, while a negative test will only have a visible control line. Invalid tests will show only the test line or no lines.

 

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What are the advantages of antigen testing for COVID-19 (compared to RT-PCR)?

RT-PCR is the gold standard of diagnostic testing for COVID-19, but antigen testing has some advantages.

  1. Faster
  2. Less expensive

Antigen test strips for SARS-CoV-2. RT-PCR tubes. Illustration.

Figure 4. Antigen testing to diagnose COVID-19 is faster and less expensive than reverse transcription polymerase chain reaction (RT-PCR).

Faster

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.

Less expensive

Antigen testing is also less expensive compared with RT-PCR. Many of the raw materials needed for RT-PCR are in shortage. Apart from RT-PCR testing supplies, personal protective equipment, and personnel training are all essential aspects of total cost and capacity for testing. If people can one day test 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. 

 

What is the main disadvantage of antigen testing for COVID-19?

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—true positives plus false negatives—times one hundred. And specificity is equal to true negatives divided by all non-diseased—false positives plus true negatives—times one hundred. A test with high sensitivity will have few false negatives and a test with high specificity will have few false positives.

Table comparing disease state to test results. Equations for sensitivity and specificity of a diagnostic test.

Figure 5. These equations can be used to calculate the sensitivity and specificity of a diagnostic test. A highly sensitive test will have few false negatives while a highly specific test will have few false positives.

Antigen tests do have 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-2. 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 number of true positives will be low when using antigen tests compared to using RT-PCR. So, the antigen tests 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-2 developed by a Belgian company.1 While the specificity of the test was high, at 99.5%, the sensitivity was quite low, only 57.6%. This means that among the diseased population, the test can only detect 57.6% of the positive cases. A large percentage of diseased patients may not be captured with the antigen test. 

Table comparing disease state to test results. Sensitivity and specificity of antigen tests.

Figure 6. Antigen tests have high specificity but low sensitivity, resulting in many diseased patients not being identified (Adapted from Mertens, P., De Vos, N, Martiny, D., et al. 2020).

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.2 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 antigen tests, a negative result will not rule out COVID-19 infection, because it may correspond to a false negative. That said, in 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, and a negative test reduces the likelihood of them 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 are transmitting the virus to others. 

 

So, when should antigen testing for COVID-19 be used?

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 exposures, history, and the presence of clinical signs and symptoms consistent with COVID-19.

RT-PCR is still the gold standard diagnostic tool, however, new diagnostic tools may help alleviate costs of testing and enable more widespread testing. Interpreting RT-PCR, antigen, and antibody tests together provide 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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References

  1. Mertens, P, De Vos, N, Martiny, D, et al. 2020. Development and potential usefulness of the COVID-19 AG respi-strip diagnostic assay in a pandemic context. Front Med (Lausanne). 7: 225. PMID: 32574326
  2. Azar, MM and Landry, ML. 2018. Detection of influenza A and B viruses and respiratory syncytial virus by use of clinical laboratory improvement amendments of 1988 (CLIA)-waived point-of-care assays: a paradigm shift to molecular tests. J Clin Microbiol. 56: e00367-18. PMID: 29695519

Recommended reading

  • Nelson, PP, Rath, BA, Fragkou, PC, et al. 2020. Current and future point-of-care tests for emerging and new respiratory viruses and future perspectives. Front Cell Infect Microbiol. 10: 181. PMID: 32411619
  • Zhang, J, Gharizadeh, B, Lu, D, et al. 2020. Navigating the pandemic response life cycle: molecular diagnostics and immunoassays in the context of COVID-19 management. IEEE Rev Biomed EngPMID: 32356761