The test is EU approved for use primarily by the professional health care, which is because blood sampling is crucial to the quality of the test result. If you do just the right thing during the sampling (watch the instruction film and follow the instructions carefully in the package leaflet), the test will be reliable, in accordance with the clinical studies that have been performed.
The test measures the antibodies IgM and IgG, which means that if the Coronavirus is present in the body, after three days there are also antibodies. Day 1-3 after a person is infected by the virus, there are no antibodies in the body. Day 4, there is enough for the test to be able to detect it reliably.
Most people infected with Covid-19 begin to show symptoms on day 7-14 after becoming infected. This means that the test can detect if the person is already infected before the person has any symptoms. When the symptoms begin to appear, there are a lot of antibodies throughout the body, which makes the test very reliable. This has been demonstrated in clinical trials and the safety of the test increases the longer the person carries the virus. Therefore, it can be good to test again after 5-10 days, if the test has shown negative at the first sampling, to ensure that the person really does not carry the Coronavirus.
Since antibodies remain in the body after the symptoms disappear and the person recovers, antibodies can still be detected with these tests, so you can see if the person has had Covid-19.