New York, Jun 10 (IANS): Researchers at the US' Johns Hopkins University have found that testing people for Covid-19 too early is likely to produce "false negative" results, even though they may eventually test positive for the virus.

"A negative test, whether or not a person has symptoms, doesn't guarantee that they aren't infected by the virus," study researcher Lauren Kucirka from JHU said in a paper, published in the journal Annals of Internal Medicine.

"How we respond to and interpret a negative test is very important because we place others at risk when we assume the test is perfect," Kucirka added.

According to the researchers, one of several ways to assess for the presence of SARS-CoV-2 infection is a method called reverse transcriptase-polymerase chain reaction (RT-PCR).

These tests rapidly make copies of and detect the virus's genetic material. However, as shown in tests for other viruses such as influenza, if a swab misses collecting cells infected with the virus, or if virus levels are very low early during the infection, some RT-PCR tests can produce negative results.

Since the tests return relatively rapid results, they have been widely used among high-risk populations such as nursing home residents, hospitalised patients and health care workers.

Previous studies have shown or suggested false negatives in these populations.

For the new analysis, the research team reviewed RT-PCR test data from seven prior studies, including two pre-prints and five peer-reviewed articles. The studies covered a combined total of 1,330 respiratory swab samples from a variety of subjects including hospitalised patients and those identified via contact tracing in an outpatient setting.

From this data, the researchers calculated daily false negative rates, and have made their statistical code and data publicly available so results can be updated as more data are published.

The researchers estimated that those tested with SARS-CoV-2 in the four days after infection were 67 per cent more likely to test negative, even if they had the virus.

When the average patient began displaying symptoms of the virus, the false-negative rate was 38 per cent. The test performed best eight days after infection but even then had a false negative rate of 20 per cent, meaning one in five people who had the virus had a negative test result.

According to the researchers, ongoing efforts to improve tests and better understand their performance in a variety of contexts will be critical as more people are infected with the virus and more testing is required.

"The sooner people can be accurately tested and isolated from others, the better we can control the spread of the virus," the authors wrote.