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How accurate are coronavirus tests? Doctors raise concern about ‘false-negative’ results

Some health experts have suggested the rate of false-negative tests could be up to 30 percent.

Christinne Muschi/Photographer: Christinne Muschi/Bloomberg

As more people are gaining access to COVID-19 tests, doctors say they are encountering a troubling number of “false negatives" — test results that indicate patients aren’t sick, despite clear signs and symptoms of coronavirus infection.

This phenomenon of “false negatives” is not new; no medical test is 100 percent accurate, but the stakes here are incredibly high. Some health experts have suggested the number of false-negative coronavirus tests could be up to 30 percent.

Doctors are worried that inaccurate results may give patients a false sense of security, leading them to become lax about hygiene and distancing, and possibly spreading more infection. Just as pressing, unreliable test results may place health care workers at higher risk.

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“A lot of my patients who have symptoms, who I clinically think have COVID-19, are testing negative,” said Dr. Alain Chaoui, head of Congenial Healthcare, a North Shore practice with 50,000 patients across five locations.

Chaoui, past president of the Massachusetts Medical Society, was unable to say precisely how many of his patients had received false-negative results, but that it appeared to be common. Chaoui said he is advising all of his patients who test negative but appear to be infected to assume they have the disease. He tells them to quarantine themselves until they experience no symptoms for at least 72 hours.

Dr. Richard Ellison, an epidemiologist at UMass Memorial Medical Center in Worcester, said medical staff there, too, are adjusting their practice out of concern about false negatives. The hospital is now retesting patients who received a negative test, but have symptoms of the novel coronavirus, Ellison said.

Doctors in Massachusetts and across the country are citing recent Chinese research that indicates the rate of false negatives there to be around 30 percent with a type of test known as RT-PCR. The test detects virus particles in nasal secretions from swabs inserted far back in the nose and is the same type widely used in the United States. The data is limited and no peer-reviewed analysis has been done in this country.

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Still, the research from China has sparked widespread interest among US doctors trying to battle the surging pandemic, while reconciling test results with what they are seeing in patients.

Dr. Harlan Krumholz, a professor of medicine at Yale and director of the Yale New Haven Hospital Center for Outcomes Research and Evaluation, raised concerns about false negatives in an opinion piece earlier this week in The New York Times.

“We need to improve the tests and to not be so confident,” he said Wednesday in an interview with the Globe.

“It’s a truism in medicine you need to look at the whole picture and not overweight one result,” he said. “We need to be detectives.”

But a top doctor for Massachusetts’ health lab expressed confidence in the tests widely used by public health departments. Dr. Larry Madoff, medical director of the Bureau of Infectious Disease at the Massachusetts health department, said the test issued by the Centers for Disease Control and Prevention are very sensitive in detecting the virus, with a sensitivity of greater than 95 percent.

“No test detects every case and there is no current ‘gold standard’ to compare it to,” he said in a statement. “Testing may be falsely negative if the test is obtained too early or too late compared to infection, or if the sample isn’t obtained or processed correctly.”

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Other doctors have said the long swabs inserted far up a patient’s nose could miss the virus if the patient is not showing many symptoms at the time of the test.

During the first days of the US outbreak, the federal government insisted that state labs use a CDC-developed test, instead of one already approved by the World Health Organization. That CDC test turned out to be flawed, and the United States lost precious weeks in tracking and tackling the coronavirus.

With a critical lack of testing, the US Food and Drug Administration shifted into high gear in late February and allowed companies to produce and release tests under an Emergency Use Authorization, and without standard federal oversight.

As questions mounted over the accuracy of tests, the FDA earlier this week released a statement saying it has provided “unprecedented flexibility" to labs and manufacturers to develop and offer COVID-19 tests. The agency noted that it had empowered states to take responsibility for the tests developed and used within their borders without FDA review.

“Every action the FDA has taken during this public health emergency to address the COVID-19 pandemic has balanced the urgent need to make diagnostic tests available with providing a level of oversight that ensures accurate tests are being deployed,” the statement said.

In Massachusetts, about half of the tests are being processed by Quest Diagnostics, a commercial laboratory based in New Jersey that is running COVID-19 tests in 12 locations nationwide, including Marlborough Mass.

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In response to questions from the Globe about false negatives, Quest said in a statement that while the tests are “considered generally accurate, no lab test is 100 percent perfect.”

The company said a doctor should consider the test results together with a patient’s symptoms and medical history.

“This includes for COVID-19,” the Quest statement said. “Our materials for providers make clear that while these tests are designed to minimize false negatives and false positives, such results can occur.”


Kay Lazar can be reached at kay.lazar@globe.com Follow her on Twitter @GlobeKayLazar. Andrew Ryan can be reached at andrew.ryan@globe.com Follow him on Twitter @globeandrewryan.