Although RT-PCR tests are the gold standard for the diagnosis of the COVID-19 coronavirus, rapid antigenic tests may represent a valid alternative, in specific circumstances.
Published on
10 Nov 2021
Although RT-PCR tests are the gold standard for the diagnosis of the COVID-19 coronavirus, rapid antigenic tests may represent a valid alternative, in specific circumstances.
Myth: The best-performing test is always the best choice
Fact: When it comes to containing epidemics, the speed and availability of tests can be important, too
What types of COVID-19 tests can we do at home?
Laboratory tests for the detection of the COVID-19 coronavirus are based on molecular methods such as RT-PCR, which detect the viral genetic material. Rapid antigen tests (RATs), instead, detect one or more proteins (called antigens) that are specific to the Coronavirus. The available at-home tests for COVID-19 are RATs that can be self-administered by any person, even without training. Depending on the version, these tests are done on a nasal swab or a saliva sample.
Myth: A patient with a positive RT-PCR is always contagious
Fact: RT-PCR can be positive also in people who are not (yet) contagious
What are the pros and cons of rapid antigen tests?
RATs are less sensitive than RT-PCR. While RT-PCR can detect even a few viral particles, RATs will give a positive result only in patients with a higher viral load. On the other hand, RATs are less expensive and provide results in about 10-15 minutes, while with RT-PCR it usually takes one or more days to get back the results from a laboratory.
Is it fair to say that RT-PCR is more accurate than RATs?
In terms of pure technical performance, yes. Experts agree that RT-PCR is significantly more accurate than the best RATs available, which makes it the golden standard for diagnosis. In practical terms, however, time is also essential. There are circumstances in which obtaining immediate result with a RAT at home, school or work is more useful than getting a very accurate result days later.
Myth: Rapid antigen tests are a just a cheaper, less reliable alternative to RT-PCR
Fact: Sometimes rapid antigen tests can be the best choice, even if they are less accurate.
How accurate are RATs in identifying and isolating infectious people?
Early identification of infectious people depends on the sensitivity of test. Soon after an infection, the viral load in patients is low: at this stage, it can be detected only with RT-PCR- which is more sensitive- and not with an antigen test. However, most patients are not yet contagious at this time. Later on, the viral load increases and patients become contagious. When the viral load is high enough to be contagious, state-of-the-art RATs are usually able to detect the virus, just like RT-PCR, and they provide an immediate answer. Therefore, these tests can be used to screen for contagious people before they can infect others, even if they are less sensitive than RT-PCR.
More generally, RT-PCR answers the question: “does a patient have the virus?” and give a positive result even with low viral loads, while RATs will light up only when a patient has enough virus to be contagious.
Where are RATs more useful?
Which test is more suitable depends on the circumstances RT-PCR is the gold standard for the diagnosis and confirmation of COVID-19 and it’s considered the best option when time is not essential. Self-administered RATs, which are faster and relatively inexpensive, can be a very practical tool for monitoring families, communities, students and workplaces and break the transmission chains, especially when repeated regularly. Therefore, some Countries are offering free at-home test and promote their widespread use. Having at-home, inexpensive kits also means that many more people can get tested, and more often, than with molecular tests alone.
