Can the time be negative 1

SARS-CoV-2: Early smears often lead to false negative results

/ picture alliance, Cedric Jacquot

Baltimore - The smear test with the reverse transcriptase polymerase chain reaction (RT-PCR), which is often carried out on contact persons to exclude an infection with the new coronavirus SARS-CoV-2, falls in the phase before the occurrence of COVID-19- Symptoms often false negative, such as an examination in the Annals of Internal Medicine (2020; DOI: 10.7326 / M20-1495) shows.

Virus production is slow to get going at the onset of a respiratory infection. It is also possible that only individual regions of the throat are affected. This reduces the chance of finding the virus genes in a smear. In practice this means - even with technically reliable tests such as RT-PCR - a high rate of false-negative results.

A team led by Lauren Kucirka from Johns Hopkins University in Baltimore evaluated the data from 7 previous studies. These included 7 preprints and 5 peer-reviewed articles. In the studies, 1,330 smears were taken at different stages of the infection.

Viruses were never detected in the smear on the first day of the suspected infection: the rate of false-negative results was 100% with a 95% confidence interval of 100 to 100%. On the fourth day of infection, the false negative test rate decreased to 67% (27 to 94%). On the day of the first symptoms, the mean false-negative rate was still 38% (18 to 65%).

According to the study, the optimal time for detection of an infection is day 8 after infection (which is usually day 3 after the onset of symptoms). Here, too, the false-negative rate is still 20% (12 to 30%), so that, in Kucirka's opinion, it is not advisable to rely solely on the result of the smear test for the diagnosis.

As early as day 9 there was an increase in false negative tests to 21% (13 to 31%). On day 21 it was again 66% (54 to 77%). Once the patient is over the infection, viruses will no longer be produced. In principle, it should then no longer be contagious. However, this only applies to the removal location. It cannot be ruled out that the patient may pass on the virus in other ways (for example through excrement or sexual intercourse).

The risk of overlooking an infection also depends on the spread of the virus in a group. If the pre-test probability is high, an infection must be expected more often even if the rate of false-negative results is low. © rme /