You started testing medical staff at your clinic early on and are supplementing these tests with a study. What is the aim of this study?
Little is known about the best strategies in hospitals to protect healthcare workers. The study aims to find out which organizational measures can effectively contribute to protection. At the University Hospital for Visceral Surgery and Medicine we treat many vulnerable patients with autoimmune diseases or immunosuppression – for example after organ transplantation. To protect them, we began intensive employee testing twice a week in March 2020 as part of an SNF project. We are still carrying these out today.
We have carried out around 10,000 tests to date and have been able to identify a relevant number of asymptomatic, mostly younger employees as SARS-CoV-2 carriers, particularly before and during the first wave. In "calmer times" the rate of positive tests was zero for many weeks. By contrast, ten people tested positive in just one week at the peak in mid-November. Overall, the rate of positive tests was 0.4% over the past year, which is not surprising given the low pre-test probability.
You talk about pre-test probability – what do you mean by that?
The pre-test probability tells us the likelihood of diseases being present before a test is carried out. If the pre-test probability is very low then you are looking for the famous needle in the haystack. It is not a precise instrument, but has to be determined on the basis of the circumstances according to criteria. With a greater pre-test probability, you are more likely to find a positive result with the given sensitivity of the test.
In the context of SARS-CoV-2 tests, there is a lot of talk about sensitivity, the sensitivity of the tests. Why?
The sensitivity indicates the percentage at which infected individuals are actually detected by using a test. Therefore, the more sensitive a test is, the more reliably it detects infected people. High sensitivity is required above all in cases where a screening procedure is used to ensure that no infected person is missed. In the case of the common PCR tests for SARS-CoV2, the sensitivity is up to 98%, which is a very high value.
By contrast, specificity refers to the probability that healthy individuals who are not infected with the virus are actually identified as healthy or negative in the test. In a screening test that aims to detect as many people as possible who are affected by a particular disease, we accept a somewhat lower specificity as long as the sensitivity is as high as possible.
Does the precautionary testing of healthy or asymptomatic people actually make sense or does it rather give a false sense of security?
Because SARS-CoV-2 is easily transmitted through social contact and symptoms vary greatly, testing is an essential part of tackling the pandemic. In this sense, testing is not a carte blanche for the individual but one component of an epidemiological concept for pandemic control and a prerequisite for detecting and interrupting chains of infection. Testing in no way replaces the usual protective measures. We cannot speak of "testing" as a general term either but must be very discriminating about the personal objective. It is important that attention is paid to high quality, methodologically correct and appropriate procedures and epidemiologically meaningful indications in all situations.
The university sees the use of saliva PCR testing as an additional way to increase the safety of students and lecturers at events that are allowed to take place with attendance due to the federal measures. However, they are not a measure to offer more attended events - which would not be allowed at the moment either.