The main coronavirus test that is supposed to tell if a person is infected or not produces false positives at least 20 percent of the time, a study published in Annals of Internal Medicine by researchers at Johns Hopkins University read.
The reverse transcriptase polymerase chain reaction (RT-PCR) test for COVID-19 has been used on high-risk persons, such as people exposed to the already infected and health workers. The research, however, suggests that the test has been producing false negatives at least 20 percent of the time, with the likelihood of false positives increasing if the test was administered in the early days of infection.
Typically, it is not until day five that symptoms for the coronavirus begin to appear. On day one of infection, the test was completely ineffective in determining if someone has coronavirus, by day four the test produced false negatives just over two-thirds of the time, at 67 percent.
After symptoms begin to appear on day five, the test was found to give false negatives 38 percent of the time. By day eight, the number of false negatives falls to 20 percent, before rising again to 21 percent on day nine, and 66 percent on day 21.
“The false-negative rate was minimized eight days after exposure—that is, three days after the onset of symptoms on average. As such, this may be the optimal time for testing if the goal is to minimize false-negative results,” the researchers wrote in the study.
The researchers also warned that the results had their own drawbacks. The increase in false negatives starting at day nine may simply be the result of infections clearing, rather than the test falsely noting that an infected person did not have the virus.
“If the goal is to clear a patient from isolation, these negative results may be correct, although more data are needed given studies showing viral replication in other sites,” the researchers wrote.
“However, if the goal of the test is to evaluate whether additional follow-up is needed or whether the patient should be treated as SARS-CoV-2–positive for the purpose of contact tracing, the test may not be providing the desired information and caution should be used in decision making,” they added.
The researchers called for health workers to consider waiting one to three days after symptoms have begun to test for the virus to minimize the probability of false negative results.