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How do rapid tests work?

At Furtwangen University, research is being conducted into the development of various rapid test procedures. One of them could provide clarity about Long COVID.

How do rapid tests work? (I20980)

At HFU, rapid tests are being developed that can indicate much more than "negative" or "positive": the fluorescent fields correspond to marked bound target molecules providing more detailed information. Image: Simone Rentschler

Just a few drops of liquid and a short wait, and you’ve got your result: during the coronavirus pandemic we have become familiar with the use of rapid tests. But how do they actually work? While the test person is waiting, capillary forces are at work inside the plastic cassette, drawing the liquid through the test strip. Tiny gold particles, which hold antibodies as "capture molecules", have been attached to the cellulose. If a SARS-CoV-2 virus comes into contact with these, this triggers the optical "alarm" - the metallic tag alters the light refraction and the test strip turns red.

"Self-tests for home use will be the way of the future!" says Prof. Dr. Hans-Peter Deigner, who conducts molecular biomedical research at Furtwangen University, including on the development of a wide variety of rapid test procedures. "It is important that the tests are carried out exactly according to instructions," adds doctoral student Isabel Quint, who specialises in the area of nanopores.  If the swab is inserted too superficially during the coronavirus test, or is not left long enough in the liquid, the test results may be inaccurate. Such test sets must be very easy to use and as reliable as possible - and "if in doubt, they should give a false positive result rather than a false negative result," explains Deigner. He and his team are currently developing a rapid test for Long COVID: "This would make it possible to determine immediately whether a patient's symptoms are the result of a coronavirus infection. Faster detection means faster treatment.

Put simply, the first step is to find a suitable biomarker, i.e. a change that can clearly be linked with the disease to be tested. This could be unusual ECG patterns, bacteria or, as in the case of COVID-19, proteins – it must be something measurable which can ideally be tested in a minimally invasive manner. The next step is to assign a target molecule to this marker which reacts to it. There are also many options when it comes to reading the tests. Deigner and his team have developed tests that are evaluated under fluorescent light: "A large number of small luminous fields which correspond to marked bound target molecules not only give a basic negative or positive indication, they also provide more detailed information," says Simone Rentschler, a doctoral student in the group.

Prof. Dr. Deigner researches rapid tests with very different objectives. He has already been working for ten years on the development of a procedure that can be used to test infants who have suffered oxygen deficiency at birth. "Certain treatments can be administered, such as lowering the baby's body temperature," Deigner explains. "However, not all affected children respond to this, and since such a treatment can also cause harm, the use of a rapid test to clarify whether it is likely to successful in individual cases is important."

A further area of research carried out by Deigner and his team, in collaboration with Prof. Dr. Matthias Kohl, is a rapid test for transplant patients, who would also be able to use it themselves on a regular basis to check the risk of organ rejection. "This is especially important in a particular phase after transplantation. Here, too, the aim is to find a marker that correlates with rejection symptoms," says Isabel Quint. Clinical trials for the tests are already underway. Their use could prove invaluable in early detection.