Scientists called the amount of false negative test results for COVID-19 by day

A group of scientists from the School of Public Health and the School of Medicine at Johns Hopkins University presented a study where they evaluated the chances of a false-negative COVID-19 test result from days of infection and onset of symptoms.

PCR tests for SARS-CoV-2 based on reverse transcriptase polymerase chain reaction (RT-PCR) are used to detect infected individuals. However, it is important to understand how the value of the test changes over time to avoid false negative results.

To establish this, a team of researchers examined data on the effectiveness of RT-PCR depending on the moment of onset of symptoms or infection with SARS-CoV-2.

Naturally, in the early stages, the accuracy of the test was low. During the period from exposure to the virus until the onset of symptoms (fifth day), the probability of a false negative result in an infected person decreases – from 100% on the first day to 68% on the fourth day. And on the day of the onset of symptoms, the possibility of false negative results was 38%.

On the eighth day, the indicator decreased to 20%, but then increased – from 21% on the ninth day to 66% on the 21st day.

The risk of receiving a false negative result was minimal on the eighth day after infection, that is, three days after the onset of symptoms on average.

The results also depend on how the sample was stored and whether the analysis itself was taken correctly.

“We found an increase in a false-negative result that began nine days after exposure; however, perhaps some of the more recent results were not actually false negative. The result at a later stage of the clinical course depends on the purpose of the test: if it is to remove the isolation of the patient, then negative results may be correct, although more data are needed, given studies showing virus replication elsewhere. However, if the purpose of the test is to assess whether additional follow-up is required, then the test may not provide the desired information and decisions should be made with caution. “

The authors believe that due to the fact that the patient may develop antibodies, combined testing is the most effective.