There are many ways to protect yourself and others from Corona. Testing at home is one of the essential prophylactic factors, along with vaccination and wearing masks.
Numerous manufacturers advertise high-quality antigen tests. However, for the test to be reliable at home, a number of factors must be correct. But how can laypeople recognize good tests, and how reliable are they?
Antigen tests for Corona
In addition to using FFP2 masks, antigen testing is one of the essential measures to detect existing corona infections and reduce the risk of infection. A distinction is made between rapid tests for professional use and self-tests for home use (1).
1. Rapid antigen tests or PoC antigen tests (PoC; point of care/near-patient laboratory diagnostics) are to be performed by trained personnel and are frequently used in hospitals, schools, or care facilities.
2. In Antigen self-tests (lay or corona self-tests), sample collection and evaluation are designed to be simpler, making them suitable for self-testing by private individuals.
Both text systems detect specific coronavirus proteins in a secretion sample from the mouth, nose, or throat and provide rapid results. However, they require a higher viral load to detect infection.
In case of doubt, a repetition of the antigen test is recommended. Positive tests should also be verified with a PCR test.
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The differences between antigen tests and PCR tests
PCR tests (PCR; polymerase chain reaction) are still the medical gold standard for corona detection. Since the evaluation is performed with special laboratory equipment, they are not available as self-tests. In contrast to the antigen test, which detects specific virus proteins, the PCR test detects the coronavirus’s genetic material (RNA).
Da die PCR das Virusmaterial in mehreren Zyklen vervielfältigt, sind geringe Virenmengen ausreichend. Der Ct-Wert (Cycle-Threshold-Wert) gibt die Anzahl der Messzyklen an, die die PCR durchlaufen muss, um das Virus nachzuweisen. Je geringer der Ct-Wert, desto mehr Virenmaterial enthielt die Probe.
Since the PCR amplifies the virus material in several cycles, small amounts of virus are sufficient. The Ct value (cycle-threshold value) indicates the number of measurement cycles the PCR must run through to detect the virus. The lower the Ct value, the more viral material the sample contained.
The PCR test is significantly more time-consuming in terms of equipment and time. It is not uncommon for individuals to have to wait between 24 and 48 hours for the result. However, the PCR test provides the most reliable information about an existing infection.
How to recognize a high-quality antigen test
In addition to pharmacies, drugstores, supermarkets, and discounters have long offered numerous Corona antigen tests for home use. Because further measures must be taken in the event of a positive rapid test, the reliability and quality of the test are essential.
Although all manufacturers advertise a sure guarantee, the quality of the test can be determined primarily based on two parameters, namely sensitivity and specificity. The sensitivity indicates the proportion of people with the disease who are identified as having the disease (positive test result). Specificity is the proportion of healthy individuals for whom the test delivers a (correctly) negative impact.
The closer these two values are to 100%, the more accurate and higher quality the selected test system is (2). Approved rapid tests and their evaluation are included in the Corona Antigen Rapid Test List (3). Sensitivity, specificity, and the viral load (Ct value) necessary for the test are used for orientation.
Alternatively, Odemshop.com offers a search box where you can enter the product name of the antigen test. You will get all available information about the product with just one click.
How reliable is a high-quality antigen test?
Wie bereits erwähnt, sind Empfindlichkeit (Sensitivität) und die Spezifität die wichtigsten Kriterien für die Testqualität. Diese beiden Kenngrößen muss der Testhersteller für die erforderliche CE-Kennzeichnung experimentell ermitteln (4). Je höher also Sensitivität und Spezifität, desto höher auch die Zuverlässigkeit.
As mentioned, sensitivity and specificity are the most important criteria for test quality. The test manufacturer must determine these two parameters experimentally for the required CE marking (4). The higher the sensitivity and specificity, the higher the reliability.
Liegt die Sensitivität bei 98 %, werden 98 % aller Erkrankten mit dem Test detektiert (= richtig-positiv). Jene 2 % der Erkrankten, die vom Test nicht erkannt werden, zeigen ein falsch-negatives Resultat (5). Ist ein Test zu 99 % spezifisch, erkennt er 1 % der Gesunden als nicht gesund an (= falsch-positiv).
If the sensitivity is 98 %, the test detects 98 % of all patients (= true-positive). The 2 % of patients not seen by the quiz show a false-negative result (5). A 99 % specific test will detect 1 % of healthy persons as not healthy (= false-positive).
Das Paul-Ehrlich-Institut setzt als Mindestkriterien für Antigentests eine Sensitivität von > 80 % und eine Spezifität von > 98 % voraus (4). Das heißt, der Test muss
The Paul Ehrlich Institute requires a sensitivity of > 80% and a specificity of > 98% as the minimum criteria for antigen tests (4).
This means that the test must,
1. 80 % of the samples with a very high viral load (Ct value ≤ 25) are positive and
2. Negative in 98% of samples that do not contain viral material.
Most antigen tests are designed for a very high viral load. The lower this is, the more likely it is that the result will not be correct. The test result may also be incorrect if the antigen test is not performed according to the manufacturer’s instructions.
A correctly performed swab is crucial to obtain sufficient viral material. If the swab is not inserted deep enough into the nose or throat, or if too little secretion is collected, the result may be falsified.
Furthermore, reading off the result after an exactly observed reading time is crucial, and evaluating it reliably is crucial. In the case of weak test reactions, evaluation can be difficult for laypersons. Despite these factors, testing in the private sector is a good option for virus containment.
Im Zweifelsfalle bzw. bei uneindeutigem Testergebnis ist ein zweite Antigentestung anzuraten. Laut einer rezenten Studie kann ein zweiter Antigentest zu einer sehr genauen Diagnose einer Coronainfektion (6).
A second antigen test is recommended in case of doubt or inconclusive test results. According to a recent study, a second antigen test can lead to a very accurate diagnosis of corona infection (6).
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Are high-quality antigen tests reliable in mutations and in vaccinated and recovered individuals?
Since virus mutations and changes are constantly occurring, the test systems for Corona are continuously optimized. Most products, therefore, deliver correct results even with altered viruses. The protein seems to be the decisive factor here.
Persons who have been vaccinated several times or those who have recovered already have antibodies. This can lead to inconclusive test results in some antigen tests, especially at the beginning. In principle, the viral load is highest at the beginning of the disease, but neutralizing antibodies could impact the viral load or the course of the disease (7).
Especially then, observing the symptoms further and confirming them with another rapid test or a PCR test is essential. In principle, however, high-quality antigen tests are an excellent way to detect an active corona infection.
Wichtig: In Deutschland zugelassene Covid-Impfstoffe sorgen nicht für einen positiven Antigentest. Die meisten Tests reagieren nicht auf das Impfstoffantigen (Spikeprotein), sondern auf das N-Protein des Coronavirus. Darüber hinaus ist das Spikeprotein nicht in der Nasen-Rachen-Abstrichprobe enthalten (4).
Necessary: Covid vaccines licensed in Germany do not provide a positive antigen test. Most tests do not react to the vaccine antigen (spike protein) but to the N protein of the coronavirus. Furthermore, the spike protein is absent in the nasopharyngeal swab specimen (4).
Conclusion
Antigen tests are a rapid and reliable method for detecting acute corona infection. High-quality test systems are characterized by high sensitivity and specificity. The closer these are to 100%, the higher the quality of the test system.
In some cases, false-positive or false-negative results may occur. A second test run can significantly increase the reliability of the antigen tests. In case of doubt or low viral load, a PCR test is recommended.
FAQ
Which self-tests are reliable?
Those who perform a self-test should pay attention to a few criteria: In addition to the CE marking and a four-digit number, the test should have a high sensitivity and specificity.
How sensitive are antigen tests?
Antigen tests for Corona vary in sensitivity. In individuals with symptoms, sensitivity is highest during the first week of illness, as viral loads are higher.
What is the difference between a rapid test and a self-test?
Antigen self-tests (lay self-tests, self-tests) function in the same way as rapid antigen tests but are more user-friendly. Rapid tests (PoC tests) should only be performed by trained personnel.
Is a rapid test safer than a self-test?
When adequately sampled and tested, self-test results are as reliable as rapid antigen tests trained personnel perform.
