Kinds of tests
When testing for the virus, one can either test for the presence of the virus or the presence of an immune response to the virus. The testing methods that look for an active virus in the body, are called PCR-tests and antigen tests.
For a virus to be active, it must have started spreading in the body. This means that if the test is taken too early after a person has been exposed to the virus, the virus might not be detected. Moreover, the virus can sometimes be detected in the body long after a person has recovered from the disease.
The PCR-tests and antigen tests therefore say nothing about how ill or infectious the person is. PCR-tests are in general costly, labor intensive, and are mostly performed at hospitals and in other health care settings. In the future antigen tests might be developed into point-of-care tests that can be used locally
The testing method that looks for the immune response to the virus is called an antibody test. Whenever a person has been infected by a virus, the individual’s immune system forms several different types of antibodies. However, the rate at which they form, varies between people. This means that an antibody test can be used to confirm that a person has had the Coronavirus, but it is unreliable during the first 8-10 days of the disease. Furthermore, some antibody tests will react to antibodies formed to other Coronaviruses than the one that produces COVID-19. The medical community therefore never uses a positive antibody test alone to confirm COVID-19. An additional PCR-test is needed.
Quality of tests
Currently, only antibody tests are commercially available as rapid tests. PCR-tests can be taken locally, but the swab must be transported to and analyzed at a laboratory, Gard notes. Antigen tests might become available as point-of-care tests in the future. Although PCR-tests are the gold standard for tests, they still miss an infection in up to 30 % of cases. This is minly because when in the course of the disease the swab was taken, errors when taking the swab, transport issues and errors at the laboratory.
Available antibody tests vary greatly in quality. In a recent evaluation of eleven rapid tests for detection of antibodies against the virus causing COVID-19, the financially independent agency The Norwegian Organization for Quality Improvement of Laboratory Examinations (Noklus) found that only 4 of the eleven tests held high enough quality to be recommended for use.
If you do want an antibody test, you have to look for a test that tests the two types of antibody called IgG and IgM. To be useful outside health care settings, the test needs to be done with a “lancet”, a small sharp needle that pricks the finger to draw a drop of blood to test. The test further needs to be specific for the type of Coronavirus that gives COVID-19. You should also choose a test that is CE-marked or has been given emergency use authorization by the Food and Drug Administration (FDA) of the United States
Which questions can the tests answer?