What Does a False Positive Coronavirus Test Result Mean?
No test is 100% accurate.
After coming under fire for trailing behind other countries when it comes to testing for the coronavirus, the US has upped its efforts over the last week and, as of March 30, an estimated 856,546 people have been tested. That figure comes from the COVID Tracking Project, an online community initiative designed to provide an accurate picture of testing in the US by collecting information from state health departments, local news reports, and live news conferences.
The testing process is simple and can potentially be done from almost anywhere. “There are few different types of COVID-19 tests available, some more accurate than others,” Gary W. Procop, MD, the chair of American Society for Clinical Pathology’s Commission on Science, Technology and Policy, and vice chair and director of virology at Cleveland Clinic, tells Health.
The recommended test is a “nasal turbinate swab test,” which essentially involves a doctor inserting a small swab inside the patient’s nose until it reaches the nasopharynx (the back of the nasal cavity).
“The CDC recently authorized healthcare workers to use alternative swabs to test for COVID-19, like the ones traditionally used for strep throat tests,” Dr. Procop says. “These swabs tend to be larger in size and less comfortable for patients since the swab must be inserted farther back into the nose.”
Some hospitals are also using blood tests to determine if patients have COVID-19, but Dr. Procop warns that these have “implications” and stresses that this type of test isn’t used at Cleveland Clinic. “Different strains of the coronavirus circulate every year, causing the common cold,” he says. “This means blood tests may cross-react and deliver a false positive if a patient has been infected by one of these viruses in the past.”
Generally, test results come back fairly quickly, depending on the test location. “Some hospitals have labs on site that can process the swabs, while others have to send them out for screening at another facility,” Dr. Procop says. “At Cleveland Clinic, we are able to test on site and share test results with patients within 24 hours. Currently, we’re averaging about an eight-hour turnaround time.”
Some newer tests that have been developed and implemented in some hospitals for people that truly need rapid results (such as pre-surgical patients) have an even faster turnaround time, he adds.
No medical test is 100% accurate, and with thousands of COVID-19 tests being administered every day throughout the world, reports of false negative results are to be expected.
A study published February 12 in the journal Radiology showed that five out of 167 patients tested negative for the disease despite lung scans proving they were ill. All five of them later tested positive for the new coronavirus. Plus, there are numerous anecdotal accounts to cast doubt on the accuracy of the test—such as Dr. Li Wenliang from China, who said his test results had come back negative several times before he was finally diagnosed.
“When the nasal turbinate swab test is performed correctly by a medical professional, the accuracy is similar to tests for influenza,” Dr. Procop says. According to the CDC, the sensitivities of the Rapid Influenza Diagnostic Tests (RIDTs) are approximately 50% to 70%.
There’s always room for error, Dr. Procop says—and that means a patient could get a false positive or a false negative result. However, he says, false positives are extremely unlikely: “Most tests target more than one region of the virus, and more than one target must be present to characterize a specimen as positive.” A false positive is more likely to occur from specimen-to-specimen cross contamination or specimen mix-up.
“False negatives can occur if the specimens are taken early in the disease process or if specimen collection is inadequate,” Dr. Procop says. “With an outbreak this severe, we are being extremely cautious and recommending that all patients with COVID-19 symptoms self-isolate and take necessary precautions, regardless of their test result.”
The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.
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