According to a study, the common PCR tests are often negative, even though people are infected. In the first few days after an infection, false results accumulate.
Thousands of people experience the redeeming moment in Germany every day: negative, no Sars-CoV-2 detectable. So all is well. Really? Not necessarily, is the conclusion of a study in the journal Annals of Internal Medicine has appeared. In it, researchers at Johns Hopkins University warn that the commonly used PCR tests alone are not very meaningful. In an overview article, the scientists led by medical doctor Lauren Kucirka evaluated the first seven studies on the reliability of the tests, and more than 1,300 patients were included in the tests.
Accordingly, at least 20 percent of the tests gave a result that was incorrectly negative. In fact, the patients were infected, which was shown by the fact that further tests were positive. Antibodies to the virus were also partially detected in the supposedly non-infected.
The scientists also showed that the time of sampling plays a major role in the validity of the result. In the first three days after infection, it was almost impossible to detect Sars-CoV-2 viruses. Tests carried out early were practically worthless. Smears from day four wrongly gave the all-clear in two thirds of all cases. On the fifth day – the time when symptoms typically set in – an average of 40 percent of the results were false negative. The error rate then decreased to 20 percent on the eighth day and then slowly increased again.
Even if these percentages show uncertainties, it can be concluded from the results that the best time for taking samples is around the eighth day, i.e. about three days after the onset of symptoms. And that the tests still fail relatively often. One reason for this could be that the smear was not done carefully enough. It is also possible that some people carry only a few viruses in the nose and throat. In that case, repeated testing might not provide better information.
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The authors warn against completely excluding infections based solely on a negative result. This is particularly the case if contagion is likely because the patient has had contact with infected people or has typical symptoms. The test result should also be interpreted with particular caution in the case of medical employees who can easily transfer the pathogen to particularly vulnerable people. The study did not investigate how high the error rate is if the result is positive. Previous estimates assume that false alarms are extremely rare. According to current knowledge, other corona viruses do not allow the tests to be rejected
Coronavirus: How do you deal with uncertainty?:Readers’ discussion