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What is the purpose of testing for COVID-19?

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COVID-19 tests detect either the coronavirus itself or antibodies made as part of the immune response to it. The virus test detects active infection and works best during the first seven days of symptoms. The antibody test detects past infection and works best from around ten days after the start of symptoms. Virus testing is used mainly for diagnosis of COVID-19. Antibody testing is used mainly for surveys of the extent of COVID-19 spread in the population.

The test for the virus detects the genetic material (nucleic acid) of the virus during an active infection. It is also called a PCR (polymerase chain reaction) test or nucleic acid test, and is sometimes inaccurately called an antigen test. The other test detects the presence of antibodies as a marker of past infection. Each test is useful in different ways and in different circumstances.

Testing during the course of infection

The coronavirus can be detected in swabs taken from the nose or throat, even before symptoms of illness first appear. This is why people can be infectious for a few days before the onset of symptoms, which usually appear about 5 days after the virus infects. Some people have no symptoms throughout their infection but are positive on the virus test and infectious to others.[1] After a few days of illness the concentration of the virus falls (as seen in swab samples) and the symptoms of the illness usually recede, while the level of antibodies in the blood goes up.[2]

An accurate test is both sensitive and specific. A sensitive test gives a positive result for a high proportion of people who are infected, so gives a low number of false negative results. A specific test gives a negative result for a high proportion of people who are not infected, so gives a low number of false positive results.

A chart showing that the nucleic acid test can be used in the first seven days, when symptoms are starting and the virus is accumulating in the airwaves. As the immune system works, less of the virus is detectable. The antibody test can be used after 7-10 days, when antibodies are increasing in the blood. We don't yet know how long antibodies last, or how protective they are. Both tests might be unreliable between days 7-10.
Chart: The rise and fall of the virus in the airways and antibodies in the blood for 21 days after exposure to infection

Virus test

The virus or nucleic acid test (see chart above) is useful mainly during active infection, before the virus is cleared by the immune system. This test is mostly used for diagnosing active infection in individuals suspected of having COVID-19, especially people with symptoms of the illness (see table below). If a virus test is positive, it is almost certainly correct (very specific). However, the test may not always detect the virus when it is present (less sensitive). In a person with COVID-19, the test is less likely to give a positive result late in the course of infection once the virus has been cleared by the immune system, or if an inadequate sample is taken from the nose or throat. A virus test normally takes an hour or less but must be done in a specialist laboratory, which can delay the result.

For an individual with symptoms, a virus test will help them get the right care, especially if clinicians are unsure if they have COVID-19 or another disease that mimics it. Knowing an individual has the illness also means they can be instructed to self-isolate at home to protect their friends, family and other members of the public from getting infected. If they are seriously ill, they can be treated in hospital with precautions to protect healthcare workers. Rapid diagnosis of individuals using the virus test can also allow tracing and isolation of people with whom an infected person has come into contact (see table). This has been done extensively in South Korea and in other countries in East Asia.

Testing for the virus flowchart

Antibody test

The antibody test (blood test) will only detect infections after the immune system has produced antibodies that recognise the virus. This happens approximately 7–10 days after symptoms develop.[2]

There is substantial work ongoing to develop accurate antibody tests for coronavirus infection. To be useful in reporting past infection and immunity in individuals, these new tests must fulfil three criteria.

First, if some people do not produce detectable amounts of the antibody used in the test after infection,[3] or if antibody levels wane with time, the test will be insufficiently sensitive, missing too many infections (false-negative results) to be useful. Second, the test must be specific enough to give high confidence that a positive test result is correct (not a false positive). This is especially true if past infection is uncommon among those who are tested, as in the general population. Infection is likely to be more common among people at higher risk, such as health workers. Third, before an antibody test can be used to indicate that someone is immune to further infection, the level of protection must be demonstrated in experimental trials. While there is a clear link between the presence of the antibody and protective immunity for many common viral infections, this has not yet been confirmed for the new coronavirus. It is possible that people are temporarily protected against reinfection but the protection wanes with time, or that protection operates against current but not future strains of the new coronavirus.

For all these reasons, antibody tests are likely to be most useful for studying past infection in whole populations to assess the scale and spread of the COVID-19 outbreak (see table below).

Testing for antibodies flowchart
Table: Testing for COVID-19. Virus testing is used mainly for diagnosis of active infection. Antibody testing is used mainly in surveys to assess the extent of COVID-19 spread in a population.
Group of people tested Which test? Results Meaning Risk of infecting others
People with symptoms Virus Negative Infection unlikelya Low
Positive Active infection confirmed High
People without symptoms but at high risk of exposureb Virus Negative Current infection unlikely Low
Positive Active infection confirmed High
Antibody Negative Past infection unlikely Low
Positive Past infection likely
Possibly immunec
Low
Sample of population for surveillance Antibody Negative No past infection Low
Positive Past infection Low
  1. Antibody tests are not routinely useful in this group because antibodies appear with a delay. But they may be useful for individuals who had a negative virus test but with symptoms persisting for more than 7–10 days, by which time the virus is no longer detectable but antibodies have begun to appear in the blood.
  2. For example, household contacts of confirmed cases, health workers or other carers.
  3. Protective immunity must be validated for each kind of antibody test.

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References

  1. Hu Z, Song C, Xu C, et al. Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China. Science China. Life Sciences. 2020 Mar:1-6. DOI: 10.1007/s11427-020-1661-4.

  2. Wölfel R, Corman VM, Guggemos W, et al. Virological assessment of hospitalized patients with COVID-2019. Nature. 2020 Apr. DOI: 10.1038/s41586-020-2196-x.

  3. Wu F, Wang A, Liu M, et al. Neutralizing antibody responses to SARS-CoV-2 in a COVID-19 recovered patient cohort and their implications. medRxiv. 2020 Apr. DOI: 10.1101/2020.03.30.20047365.

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