How Accurate Are COVID-19 Tests?

While diagnostic testing is playing a critical role in the fight against COVID-19, experts at Johns Hopkins Medicine are warning that false negatives are a very real threat. According to the team, the chances of testing negative for COVID-19 for people who are or have recently been infected with the coronavirus is more than one in five. At times, this number can be significantly higher.

“Tests for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) based on reverse transcriptase polymerase chain reaction (RT-PCR) are being used to “rule out” infection among high-risk persons, such as exposed inpatients and health care workers,” warns the study abstract. “It is critical to understand how the predictive value of the test varies with time from exposure and symptom onset to avoid being falsely reassured by negative test results.”

The dangers of RT-PCRs

Reverse transcriptase polymerase chain reaction tests, also known as RT-PCRs, are one of the most common diagnostic tools used to test for COVID-19. They rely on respiratory samples to identify viral particles and determine if a patient has contracted the SARS-CoV-2 virus. While they are useful, the Johns Hopkins Medicine team say results are not always accurate.

The findings were published in the journal Annals of Internal Medicine, with the researchers saying timing plays an important role in accuracy. On day one of infection the probability of a false negative is 100%. By day four the probability has decreased to 67%, with false negative rates dropping to 20% by day eight. This is usually when a person starts presenting symptoms. Further into the illness, false negative results increase to 21% on day nine and are back up to 66% by day 21.

“Infection should not be ruled out on the basis of RT-PCR alone” say experts

The study drew on data from seven existing studies analysing RT-PCR accuracy rates and adds to a growing body of research suggesting negative test results should be taken with a grain of salt, particularly if the patient in question is presenting symptoms or has been exposed to a confirmed case.  

“Care must be taken in interpreting RT-PCR tests for SARS-CoV-2 infection—particularly early in the course of infection—when using these results as a basis for removing precautions intended to prevent onward transmission,” reads the conclusion. “If clinical suspicion is high, infection should not be ruled out on the basis of RT-PCR alone, and the clinical and epidemiologic situation should be carefully considered.”

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