ROCHESTER, Minn. — With a growing number of COVID-19 tests coming back negative, a team of Mayo Clinic doctors cautioned against using test results as an excuse to ease up on social distancing.

Diagnostic sensitivity of the primary test for the new coronavirus — the probability a test will accurately detect the virus — is not fully known. A false-negative test could cause people infected with COVID-19 but not showing symptoms to return to work and spread the disease, according to an article published Thursday, April 9, in the peer-reviewed journal Mayo Clinic Proceedings.

A study from China cited in the article put the sensitivity of reverse transcriptase–polymerase chain reaction, or RT-PCR, testing as low as 71%. Even with sensitivity of 90%, that would mean 2 million expected false-negative results if, for example, comprehensive testing was conducted on California’s 40 million residents.

‘RT-PCR testing is most useful when it is positive,” according to Dr. Priya Sampathkumar, a Mayo Clinic infectious diseases specialist and co-author of the article. “It is less useful in ruling out COVID-19. A negative test often does not mean the person does not have the disease, and test results need to be considered in the context of patient characteristics and exposure.”

The authors further warned of false-negative results for health care personnel. Assuming a low-ball infection rate of 10% for the country’s 4 million workers providing direct patient care, testing every provider would result in more than 40,000 false-negative tests, according to the article.

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Centers for Disease Control and Prevention guidelines call for health care workers suspected to have COVID-19 to return to their job if they tested negative and are asymptomatic. False-negative tests could risk providers spreading the respiratory disease, Mayo Clinic physician and co-author Dr. Colin West said in a news release.

The authors outline four recommendations regarding COVID-19 and the potential for false-negative tests:

  • Continue strict social distancing, hand washing and disinfecting measures regardless of COVID-19 risk factors, symptoms or test results.

  • Assess a patient’s risk level for COVID-19 prior to testing and view negative tests with caution.

  • Establish protocols for managing negative results, such as delaying a return to work for high-risk individuals and health care workers, even when asymptomatic.

  • Develop more sensitive tests or combinations of tests to minimize false-negative results.

RT-PCR testing is only one factor Mayo Clinic uses to determine if a patient meets the criteria for COVID-19, Sampathkumar said. If test results are negative but chest scans show abnormalities or the person has known contact with a confirmed case, the recommendation is to care for the patient as if they have the disease.

Testing by the numbers

State health departments provided updated testing numbers for COVID-19 on Thursday. Officials noted not all suspected cases of COVID-19 are tested, so results do not represent the total number of people infected.

  • Minnesota: 1,242 positive cases out of 32,294 completed tests

  • Wisconsin: 2,885 positive cases out of ‭34,309 completed tests

  • North Dakota: 269 positive cases out of 8,990 completed tests

  • South Dakota: 447 positive cases out of ‭7,147 completed tests