Rapid Tests (57)

Detecting the virus: Different methods for rapid results

As the pandemic progresses, especially with the second wave, there is increasing interest among the public in reliable tests that can quickly detect infection. A number of methods are now available, and manufacturers are still offering additional, improved products. However, even with a rapid test, the course of the disease should be taken into account when using it. Not every test is useful at every time and in every situation. 

The PCR rapid test

Like the classic laboratory test, the PCR rapid test is based on the detection of the genetic material of the virus in the human organism. Several steps are necessary to detect the polymerase chain reaction (PCR), which is why the virus test is originally performed in a laboratory. But the mobile method, also called the cartridge test, has already integrated the laboratory, and the virus rapid test can be performed at any location by means of a swab in the oral pharyngeal cavity. 

So the test material is not transported to the laboratory with a time delay. And the results are available quickly, usually after two and a half hours. However, the PCR rapid test is significantly more expensive and additionally not particularly reliable. In case of unclear results, medicine therefore recommends further examinations.

For whom is the PCR rapid test suitable?

Because of the disadvantages mentioned above, this virus test is only recommended in certain situations. It is therefore used more frequently in outpatient clinics, emergency rooms or care facilities. The procedure does not fundamentally change the pandemic. And certain medical knowledge is necessary, which, however, can easily be acquired by the layman. For example, the Medical Service of the Health Insurance Funds (MDK) offers videos on its website for training purposes, even for patients. With the instructions presented there, it should also be possible for affected persons or relatives to take samples professionally and to prevent false results when performing the quick test themselves.

The ct value

The value for the cycle threshold (ct) indicates the number of cycles required for a positive result. In other words, the value tells us how many times the sample material had to pass certain tests until SARS-CoV-2 is reliably detected. If a result is obtained after only a short time, the physician concludes that there is a high exposure to the virus, i.e. the ct value remains low. An elevated value means that the sample had to be tested many times until the genetic material of the virus was detectable.

The evidence so far suggests that at values above 30, the virus is more difficult to grow. This further means that patients with this result have a lower infectivity. However, the test can only give an indication and is not very informative as a stand-alone method. Additionally, it is dependent on the location where it was performed, the time for transport, and the test system used. 

The antigen test

Another test method that is becoming increasingly popular is the rapid antigen test. This does not detect the genetic material, but protein fragments of the virus. The test is similar in simplicity to the pregnancy test. The sample material is applied and there is a reaction of the test unit. It is therefore easy to produce in large quantities. Results are available after only 30 minutes, making the method faster than a PCR rapid test. Depending on the manufacturer, however, there are still considerable differences in the sensitivity and specificity of the devices.

These tests are particularly useful as a supplement to the PCR test to meet previously defined requirements in specific situations. In hospital emergency rooms, for example, it is often a matter of quickly presorting potentially infected and potentially non-infected patients. The German Federal Institute for Drugs and Medical Devices (BfArM) publishes suitable tests that meet certain minimum criteria. The Paul Ehrlich Institute, in cooperation with the well-known RKI, the Robert Koch Institute, defines the relevant requirements for antigen tests.

Combination of different methods

However, laboratory medicine doubts that the rapid tests are as reliable as the laboratory variant. One of the reasons for this is that the corona viruses are very similar to each other. Sometimes, therefore, the test is positive because of another virus of the Corona family instead of SARS-CoV-2.

Therefore, additional methods are necessary. In addition, as with the PCR test, the antigen test normally requires a swab taken deep in the throat. This is not considered straightforward, yet antigen tests are able to further simplify testing. 

For the assessment of the health situation of the affected person, it is still significant that a negative antigen test does not exclude an infestation with the virus. At the beginning of an infection, when only a low viral load is present, the result may be negative. The same is true even in the second week after the onset of symptoms. The time of performance is also decisive for the significance of this method. And the test is always only a snapshot, however, a positive result speaks for a possible infection. Confirmation is obtained by a PCR test, the positive result of which results in a report to the public health department.

When is the test suitable?

According to the Ministry of Health, the antigen test is useful in certain situations for certain groups of people. If there is an increased 7-day incidence (approximately greater than 50), testing in hospitals, nursing homes or doctors' offices is recommended for nursing home residents, patients, staff and visitors. In case of outpatient surgery or dialysis, the procedure can be used on site, thus infected persons can be detected immediately. Apart from the incidence value, the testing concept of the facility is decisive, but to be on the safe side, one tests persons who do not show any symptoms. The goal of this measure is to rule out disease in the interest of the practice staff and other visitors.

The antibody rapid test

The test for antibodies indicates whether someone has already been infected without knowing it. If antibodies are detected, the immune system has already been in contact with the virus. But here, too, the results must be put into context. For example, recent results show that in some cases antibodies are no longer detectable in symptom-free infected persons. Then the immune system has fought the virus right at the beginning of the infection.

Depending on the production, reliable results can be expected in up to 99.9 percent of the tests performed. The development of antibodies in the human organism must also be taken into account with these tests. This is because their production does not begin immediately after infection, but in some cases only after three weeks. This is how long one should wait for the detection to be reliable.