The research has recently found that PCR-based tests for SARS-CoV-2 have a false negative rate of at least 20 percent, depending on the time of testing. A team of scientists from Johns Hopkins Medicine stated that the majority of tests for the novel coronavirus involve taking a swab from the back of the nose or the throat for genetic analysis. This analysis uses a laboratory procedure called reverse transcriptase-polymerase chain reaction (RT-PCR), which converts genetic material from the virus (RNA) to DNA before amplifying it. “If clinical suspicion is high, infection should not be ruled out” based on the test alone, they wrote. “The clinical and epidemiologic situation should be carefully considered.” The research team analyzed seven studies with 1,330 samples of reverse transcriptase polymerase chain reaction tests. These RT-PCR tests are typically used to “rule out” infection among high-risk people, such as health care workers or hospitalized patients who may have been exposed to the coronavirus. According to the review, the probability of a false-negative is higher in the first days of infection before symptoms typically start on day 5, they wrote. On the day of symptom onset, the false-negative rate is about 38%, which decreases to 20% on the third day after symptom onset. The third day of symptoms is likely the best day to use RT-PCR tests, they wrote. After that, the false-negative rate increases again. About three weeks after infection, the false-negative rate could be between 54% and 77%, they wrote.