Coronavirus: What you should know about tests
If the genetic material is found, the patient is considered contagious. If the genetic material is not found, however, it does not necessarily mean that the person tested is not infected after all: it is still possible that the viruses are not in the sample but are present elsewhere in the body. .
It may also explain why, in individual cases, COVID-19 patients considered cured later tested positive on PCR tests. Probably in these cases the viruses were present all the time but were not found in the test until the patients were declared cured.
In a podcast with German public broadcaster NDR, infectious disease specialist Christian Drosten of Berlin's Charite Hospital compared it to trying to catch a goldfish in a tank with a dip net. If you take the net out of the water and there are no fish in it, it does not mean that there are no fish in the tank.
PCR rapid tests Traditional PCR tests are performed in the laboratory. Most of the time, this is done via high-throughput screening, in which thousands of samples can be tested simultaneously. This procedure usually takes several hours to produce results, and patients normally have to wait between half a day and a few days to receive the results.
Rapid PCR tests are a possible solution to this time-consuming process. In this case, the test is not carried out in a central laboratory, but can be carried out on site using mobile equipment. These devices can produce a result in 45 minutes. The downside is that the devices can't handle more than 80 tests per day.
Antigenic tests These new rapid tests have only been on the market since this summer and are supposed to be as easy to use as a pregnancy test. The antigen test also involves taking a saliva sample. This approach detects viruses using the fluorescence immunoassay (FIA) method and can usually determine if a patient is severely infected and contagious within 15 minutes.
Many tests today achieve good sensitivity and above all a sufficiently high specificity of more than 98%. This means that there are relatively few false positives. The research team of virologist Christian Drosten from Charité in Berlin compared seven different types of antigen tests. Only two of them failed - reaching only between 88 and 94% specificity. The results were published on MedRXiv on Nov. 13 as a non-peer-reviewed preprint.
The main advantages, however, are quick results and the possibility of using it directly on site. Some tests require a special device for analysis.
Flu tests that work on the same principle achieve a sensitivity of just over 50% and a specificity of around 99%. This would mean that only one out of two virus carriers is detected. One in 100 people tested would have a false positive result.
Nevertheless, a growing number of doctors favor the widespread use of antigen testing because they hope it will eventually detect more infected people.
They also argue that test sensitivity increases when patients have a high viral load, which is when they are most infectious and, therefore, when they pose the greatest risk to the community.
In most countries, including Germany, these rapid tests are not available free of charge. As the Infection Protection Act states that all SARS-CoV-2 illnesses must be reported, only doctors can perform the tests.
Evidence of infection in the past Serological tests, also called enzyme immunoassays (ELISAs), detect antibodies that the immune system has gathered against the virus in the blood of people being tested. This means that the body has already shown an immune response to infection with a particular virus. For ELISAs, a person must give a small sample of blood which is then tested in the laboratory.
Manufacturers now also offer rapid tests based on this principle, but these must always be carried out by a medical practitioner. All it takes is a few drops of blood - much like a diabetes test - to be placed in a test cassette and a buffer solution added.
If SARS-CoV-2 specific IgM and IgG immunoglobulins are present in the blood, the sample changes color. A positive result may mean that the person being tested has had a coronavirus infection and now has some immunity against it.