COVID-19 test is best performed three days after the symptoms appear. Testing people too early in the course of infection may result in a false negative diagnosis, even though they may eventually test positive for the virus, reported The Indian Express (TIE) quoting a study.
The review research, published in the journal, Annals of Internal Medicine, assessed data on RT-PCR tests. These detect viral genetic material in samples from seven prior studies.
In the analysis, the researchers from Johns Hopkins University, USA analysed a total of 1,330 respiratory swab samples from a variety of subjects including hospitalised patients.
“A negative test, whether or not a person has symptoms, doesn’t guarantee that they aren’t infected by the virus,” said TIE quoting Lauren Kucirka, a co-author of the study from Johns Hopkins University.
“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. However, those infected with the virus are still able to potentially spread the virus,” Kucirka explained.
Decoding the tests
- Patients who have a high-risk exposure should be treated as if they are infected, particularly if they have symptoms consistent with COVID-19.
- Patients must be informed about the shortcomings of the tests.
- When a swab misses collecting cells infected with the virus, or if virus levels are very low early during the infection, some tests can produce negative results.
However, they said the tests give relatively rapid results, due to which they have been widely used among high-risk populations such as nursing home residents, hospitalised patients, and health care workers.
Based on previous studies, the scientists cautioned that there could be false negatives in these populations.
According to the researchers, the test performed best eight days after infection, which on average is three days after symptom onset.
But even then, they said the test had a false negative rate of 20 percent, meaning one in five people who had the virus had a negative test result.
“We are using these tests to rule out COVID-19, and basing decisions about what steps we take to prevent onward transmission, such as the selection of personal protective equipment for health care workers,” said Kucirka.
“As we develop strategies to reopen services, businesses, and other venues that rely on testing and contact tracing, it is important to understand the limitations of these tests,” she added.
The sooner people can be accurately tested and isolated from others, the better we can control the spread of the virus, the scientists said.
