Testing for COVID-19: How to best use the various tests?
Even with promising recent news on vaccine development, testing, tracking, tracing and isolating (TTTI) quickly and on a large scale continue to be essential to public health policy responses to the COVID‑19 pandemic. This note provides an update to an earlier OECD brief on such strategies in the light of recent developments in testing technologies. Molecular tests, and in particular RT-PCR, remain the reference for identifying infections because these tests are very reliable. But capacity constraints and the relatively high cost of RT-PCR limit its use on a massive scale. More recently-developed rapid antigen tests offer the advantage of producing results much more quickly. They are also cheaper, simple to use, and can be performed at point-of-care, thus allowing their use on a very large scale. However, they are less reliable than molecular tests. To achieve their objectives, testing strategies can combine different technologies and use them in complementary ways, taking into account their respective strengths and limitations.
Although the recent developments on the development of the COVID19 vaccine are encouraging, testing, tracking, tracing, and isolating (TTTI) strategies will continue to be crucial. Only TTTI can prevent future outbreaks of infections after lockdowns until the entire population has been immunized. Testing technologies can provide information about such strategies in two main ways.
- Molecular tests, and in particular RT-PCR, remain the reference for identifying active infections. These tests have proven to be very reliable – they achieve high sensitivity and specificity. But capacity constraints and the relatively high cost of RT-PCR tests limit their use on a massive scale. It also takes long to produce test results.
- Rapid antigen tests have the main advantage of producing results much more quickly. They are also simple to use, can be performed at point-of-care, and are cheaper than molecular tests, allowing their use at very large scale. However, they are less reliable than molecular tests – they achieve good specificity but only moderate sensitivity.
In selecting the appropriate type of test, the objectives of the testing strategy should be considered, taking into account the strengths and limitations of the test.
- Monitoring of specific population groups in which a new cluster of infections is suspected to occur is the most appropriate use of point-of-care rapid antigen tests, given that they can be used quickly and at scale. But this requires tests to be repeated. Testing passengers for air travel can be a means to provide additional screening before boarding flights and to loosen quarantine requirements upon arrival, but such strategies should be adopted with caution.
- Rapid antigen tests are the only workable option for massive screening campaigns in the general population. While this may appear to be an appealing strategy to guide containment measures, the related challenges should not be underestimated and effectiveness is uncertain.
- Molecular testing is the preferred option in a clinical setting to diagnose patients and inform treatment decisions, because of the reliability of these tests minimises the risk of a misdiagnosis.
- Molecular tests are also the best option for people who show symptoms and people who were in contact with a confirmed case to inform TTTI strategies. Certain point-of-care rapid antigen tests can, however, constitute a possible alternative to molecular tests in such situations. But repeated testing or confirmatory molecular tests might be necessary. The utility of point-of-care rapid antigen tests in TTTI strategies depends on whether increased speed and lower costs, and therefore more tests, can outweigh lower sensitivity.
Point-of-care RT-LAMP and CRISPR-based tests may ultimately overcome some of the limitations of RT‑PCR and rapid antigen tests. However, their development is still ongoing and they are not yet widely available. Also, the logistical implications of using these tests are not clear yet.
Despite promising recent developments regarding COVID19 vaccine development, there is still a long way to go before vaccines can be made available to the general population. Testing, tracking, tracing, and isolating (TTTI) as rapidly, massively, and intelligently as possible will continue to be essential to prevent future recurrences of infection following lockdowns. In order to suppress infections rapidly, it is vital to test suspected cases and their contacts in a timely manner to determine who is infected, isolate those infected, track them effectively in order not to spread the disease further, and list exhaustively all persons with whom they have had contact. An overview of available testing technologies has been provided in the OECD policy brief titled Testing for COVID-19: A way to lift confinement restrictions, published in May 2020. Since then, progress has been made in the development of new testing methods and repurposing of existing technologies for COVID‑19 including, among others, rapid antigen tests and other molecular diagnostic techniques (mainly CRISPR1-based tests and RT-LAMP2).
As opposed to RT-PCR, which has been used most widely up to now, new rapid antigen tests can be easily deployed at point-of-care and provide almost immediate results. These features are very useful for improving TTTI strategies but, as described below, rapid antigen tests are less sensitive than RT-PCR, and this can limit their utility in some scenarios. Point-of-care RT-LAMP and CRISPR-based tests are gaining a lot of attention and could become useful complements to the “testing tool box” in the medium term.
This note provides an update to the earlier OECD brief in the light of these recent developments in testing technologies, and discusses implications for more effective containment and mitigation strategies until vaccines become widely available.