Rapid antigen tests for SARS-CoV-2: The benefits and drawbacks

Rapid antigen tests for the detection of SARS-CoV-2 look set to become a key part of the arsenal in the fight against the Covid-19 pandemic with more than 100 companies around the world developing and manufacturing these tests. They offer a number of benefits over the gold standard RT-PCR tests, but also significant drawbacks. International Hospital looks at these rapid antigen tests in the context of the overall portfolio of tests available.

Testing is a critical cornerstone of the Covid-19 pandemic response, enabling the surveillance, contact tracing, infection prevention and control, and clinical management of Covid-19 – and to prepare for the roll-out of vaccines once available. As such, timely and accurate Covid-19 testing is essential.

Effective testing strategies rely on a portfolio of test types that can be used in different settings and situations. There are two main types of tests: diagnostic tests and antibody tests.

At the outset it is worth noting that no test is 100 per cent accurate, and test performance can vary based on the prevalence of disease in the population being tested. Covid-19 diagnostic tests may be less accurate in populations with a low prevalence of disease and in asymptomatic individuals, individuals who shed little virus, or individuals who are early or late in the course of illness.

Tests are assessed based on their sensitivity and specificity. A test’s ‘sensitivity’ is the fraction of positive cases that the test correctly identifies as positive, and a test’s ‘specificity’ is the fraction of negative cases that the test correctly identifies as negative.

  • A highly sensitive test will generally have a low false negative rate but will run a risk of false positives if the test’s specificity is low.
  • A highly specific test will generally have a low false positive rate but will run a risk of false negatives if the test’s sensitivity is low.

Antibody test

The antibody or serology test measures the number of antibodies present in the blood when the body is responding to a specific infection, like SARS-CoV-2. This means the test detects the body’s immune response to the infection caused by the virus rather than detecting the virus itself. They are important to tell if someone has been previously infected and will provide more understanding of the coronavirus, including how long antibodies stay in the body and if they provide immunity.

On 6 November, the U.S. Food and Drug Administration (FDA) authorized the first serology test that detects neutralizing antibodies from recent or prior SARS-CoV-2 infection, which are antibodies that bind to a specific part of a pathogen and have been observed in a laboratory setting to decrease SARS-CoV-2 viral infection of cells. The FDA issued an emergency use authorization (EUA) for GenScript’s cPass SARS-CoV-2 Neutralization Antibody Detection Kit, which specifically detects this type of antibody.

Although the FDA has previously issued EUAs to more than 50 antibody (serology) tests, those tests only detect the presence of binding antibodies. Binding antibodies bind to a pathogen, such as a virus, but do not necessarily decrease the infection and destruction of cells. The FDA emphasizes that the effect of neutralizing antibodies for SARS-CoV-2 in humans it is still being researched.

The Administration cautions patients against using the results from this test, or any serology test, as an indication that they can stop taking steps to protect themselves and others, such as stopping social distancing, discontinuing wearing masks or returning to work. They also reminds patients that serology tests should not be used to diagnose an active infection, as the test only detect antibodies that the immune system develops in response to the virus, not the virus itself.

To enable detection of active infection, surveillance, contact tracing and clinical management of the Covid-19 pandemic, diagnostic testing is required.

There are two types of diagnostic tests: molecular and antigen tests.

Molecular test

The molecular test uses a process called reverse transcription polymerase chain reaction (RT-PCR). Polymerase chain reaction (PCR) is a process that amplifies (replicates) a small, well-defined segment of DNA many hundreds of thousands of times, creating enough of it for analysis. Test samples are treated with specific chemicals that allow DNA to be extracted. Reverse transcription converts RNA into DNA. RT-PCR first uses reverse transcription to obtain DNA, followed by PCR to amplify that DNA, creating enough to be analysed. RT-PCR can thereby detect SARS-CoV-2, which contains only RNA. Real-time PCR (qPCR) provides advantages including automation, higher-throughput and more reliable instrumentation. It has become the preferred method.

To date, testing for SARS-CoV-2 infection mostly relies on RT-PCR performed on a nasopharyngeal specimen. This testing method remains the gold standard for detecting SARS-CoV-2 and is characterised by both high sensitivity and specificity in detecting viral RNA. However, the turnaround time can, in practice, take a few days. Diagnostic laboratories routinely perform RT-PCR tests, which require extraction of viral RNA as well as laboratory instrumentation for nucleic acid amplification and detection. Theoretically, the time required to perform the RT-PCR test is a few hours, but in practice specimens often need to be transported from the place of sampling to the laboratory, and additional time then elapses until the specimen is processed. Limited internal laboratory capacity, including trained staff, and high sample volumes, may also contribute to the delayed processing of samples. As a result, the turnaround time can easily increase to several days.

In addition, the current upsurge of Covid-19 cases in Europe, coupled with the usual rise of other respiratory infections during autumn and winter, has led to a dramatic increase in the demand for SARS-CoV-2 tests. The high volume of samples reaching the laboratories can lead to a shortage of reagents and disposables as already reported by some countries, and to a further increase in the turnaround time for RT-PCR tests. [1]

Antigen test

On the other hand, antigen tests can provide a result in 10-30 minutes, are relatively inexpensive and can be used at point-of-care. But there is a trade-off between speed and lack of sensitivity, meaning that this test runs the risk of providing false negative results.

In addition, on 3 November, the FDA issued an alert to clinical laboratory staff and healthcare providers that false positive results can occur with antigen tests, particularly when users do not follow the instructions for use of the rapid antigen tests. They urge healthcare providers to carefully read the authorized instructions for use for each test and emphasize that reading the test before or after the specified time could result in false positive or false negative results. [2]

The rapid diagnostic test (RDT) detects the presence of viral proteins (antigens) expressed by SARS-CoV-2 – most commonly the so-called spike protein on the surface of the virus – in a sample from the respiratory tract of a person. If the target antigen is present in sufficient concentrations in the sample, it will bind to specific antibodies fixed to a paper strip enclosed in a plastic casing (a lateral flow immunoassay) and generate a visually detectable signal, typically within 30 minutes. The antigens detected are expressed only when the virus is actively replicating; therefore, such tests are best used to identify acute or early infection.

A technical report published by the European Centre for Disease prevention and Control (ECDC) notes that one of the key benefits of the rapid diagnostic test for SARS-CoV-2 antigens (Ag-RDT) is that they may be sensitive enough to detect cases with a high viral load, i.e. pre-symptomatic and early symptomatic cases (up to five days from symptom onset, which likely account for a significant proportion of transmissions.

The authors of the ECDC report point out that rapid antigen tests should be applied in a way that compensates for the lower performance compared to RT-PCR, i.e. by including repeat testing for screening purposes and confirming test results by RT-PCR. Besides the performance of the test, other practical and strategic aspects play a significant role in deciding if a test can be used and with which indications. Examples of these considerations are the timeliness of test results, the scalability, the simplicity of use, instrumentation availability, human and material resources, and overall logistical arrangements for sampling and testing and costs. The epidemiological situation per setting, local area, region and nationwide also affect the testing strategy. In addition, Ag-RDTs can be used for the early detection of cases when RT-PCR testing capacities are not available.

In addition, the ECDC report notes that rapid antigen testing can be particularly helpful from a public health perspective and may be more beneficial than RT-PCR for the following:

  • prompt clinical management of cases with COVID-19-compatible symptoms at admission;
  • control transmission via: early detection of cases, contact tracing, population-wide testing;
  • mitigate the impact of Covid-19 in healthcare and social-care settings: triage at admission, early detection and isolation;
  • identify clusters or outbreaks in specific settings: early detection and isolation.

Use in Europe

A survey, conducted by the European Commission Health Security Committee in September, investigated Member States’ practices in using rapid antigen tests. Overall, of the 15 States that responded to the survey, five States are using rapid antigen tests for some aspect of the response to the Covid-19 pandemic. Nine of the 15 countries are carrying out clinical validation studies or pilots to assess the clinical/diagnostic performance and potential use of rapid antigen tests, and two Member States are not considering the use of rapid antigen tests. According to this survey, some countries use the tests as part of their testing strategy for early identification of cases, conducting contact tracing, and/or implementing rapid isolation and quarantine of detected cases and their contacts. Other countries use rapid antigen tests specifically to ensure laboratory testing in remote areas where the gold standard RT-PCR is not available or timely enough. One country uses them for testing incoming travellers from other countries and in schools. [1]

Regulatory requirements

According to the World Health Organization there are more than 100 companies developing or manufacturing Ag-RDTs. So there are a considerable number of options on the table. How do you choose which ones to use? The choice is guided by regulatory requirements and each country has developed their own, in many cases based on the recommendations of the WHO.

The WHO recommends rapid antigen tests that meet the minimum performance requirements should have ≥80% sensitivity and ≥97% specificity in a range of settings where RT-PCR is unavailable or where an excessive turnaround time would preclude clinical and public health utility of results. [4]

The ECDC recommends that tests with a performance closer to RT-PCR, i.e. ≥90% sensitivity and ≥97% specificity, should be used. [1]

According to the US CDC, the first antigen tests to have received FDA EUA demonstrate sensitivity ranging from 84.0%-97.6% and have reported specificity of 100%. Currently, the rapid antigen tests that have received EUAs from the FDA are authorized for diagnostic testing on symptomatic persons within the first five to twelve days of symptom onset. [5]

The CDC advises testing professionals who perform rapid antigen testing that these tests should be interpreted in the context of the prevalence of infection, the device’s performance characteristics and instructions for use, and the patient’s clinical signs, symptoms, and history. The CDC notes that negative diagnostic testing results should be handled differently depending on the testing device and its stated performance characteristics. “In most cases, negative antigen diagnostic test results are considered presumptive. CDC recommends confirming negative antigen test results with an RT-PCR test when the pretest probability is relatively high, especially if the patient is symptomatic or has a known exposure to a person confirmed to have Covid-19. Ideally, confirmatory RT-PCR testing should take place within two days of the initial antigen testing.”

Conclusion

In conclusion, rapid antigen tests can contribute to the overall Covid-19 testing capacity offering an advantage in terms of shorter turnaround time and reduced cost, especially in situations where RT-PCR testing capacity is reduced. However, where rapid antigen tests are used, test performance and prevalence of infection in the target population need to be taken into consideration as there continues to be a considerable risk of false negative and positive results with these tests.