The Science of testing for COVID-19

SARS-Cov-2, Coronavirus or COVID-19 originated in China in late 2019 and attacks the respiratory system causing pneumonia and potentially fatal outcomes.

It is understood that symptoms can sometimes take 14 days to appear with fever, a dry cough and fatigue as common early signs.  This makes the transmission of the disease possible even when the carrier is showing no symptoms.

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 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.

Virus test

The virus or nucleic acid test is useful mainly during active infection, before the virus is cleared by the immune system. This test is used for diagnosing active infection in individuals suspected of having COVID-19, especially people with symptoms of the illness. If a virus test is positive, it is almost certainly correct (very specific).

Antibody test

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

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