You can test negative and still have the coronavirus. Lot's of people do. Here's why

To curtail the coronavirus more quickly, Craig Blakely’s advice is to act as if you have it.

Even if your test says you'r negative.

The dean of the University of Louisville’s School of Public Health and Information Sciences is concerned that those who test negative may not be as vigilant as the virus demands.

Estimates of false negatives range as high as 40% on the coronavirus test, meaning people could feel a false sense of security.

“There’s still a plausible opportunity that you’re spreading,” Blakely said Saturday. “And you may be less inclined to be protective and cautious because you’ve just validated in your mind (as uncontaminated).

“You might stay at home, but you would be more inclined to sleep in the same bed with your spouse and play with your kids. You shouldn’t be doing that. That’s what the problem is. You should be very cautious.”

Dr. Sarah Moyer, Louisville’s chief health strategist, said Friday that research had shown a false-negative rate of about 40%.

The New York Times’ Ross Douthat used the same statistic in a Tuesday column, referencing a recent study of three Chinese hospitals that showed nasal swab testing had been able to detect only five of eight confirmed cases (63%).

Larger samples may show a lower rate of false negatives. Former Stanford University pathologist Dr. Bruce Patterson told San Francisco’s KGO-TV he expects false negatives of coronavirus to range between 10% and 15%, in line with testing for other viruses.

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“I think that’s in the right ballpark,” said Dr. Paul McKinney, associate dean for research at U of L’s School of Public Health and Information Sciences. “I would hope it’s not 15 percent, but what you’re really talking about is what is the sensitivity of the test? That is, how many of the people who are infected can you detect with the current technology?”

McKinney cautioned that drive-thru nasal swabs administered in windy or cold conditions may be less reliable than tests taken in a laboratory with a still subject.

Accordingly, individuals whose tests come back negative might be well-advised to continue exercising caution as if they were contagious.

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“If they came in and they have every classic symptom and they have been in contact with someone else who had COVID-19, and they test negative, I wouldn’t believe that as being the definite negative test,” McKinney said. “I would retest that person in 24 to 48 hours to see what’s going on. If you have multiple negative tests, that increases the likelihood that it’s for real.

“(But) A single negative test of somebody with a high probability of disease, I would not consider that to be definitive.”

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The U.S. Food and Drug Administration on Friday approved the emergency use of a coronavirus test that can provide results within 5 minutes, a test Abbott Laboratories expects to distribute at a rate of 50,000 per day beginning next week.

Another newly authorized test, this one developed by the molecular diagnostics firm Cepheid, reportedly delivers results in 45 minutes.

“Our testing capacity is significantly scaling,” Louisville Mayor Greg Fischer said Friday.

Blakely described the new tests as “hot news” and expressed confidence the additional tests would enable public officials to identify more coronavirus positives and, in turn, to isolate them.

His concern is that people may mistake testing negative for coronavirus as an “all clear.”

“You still need to be just as careful as you were being before you had any sense of wanting to get tested in the first place,” Blakely said. “Because it’s all around us.”

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Tim Sullivan: 502-582-4650, tsullivan@courier-journal.com; Twitter: @TimSullivan714. Support strong local journalism by subscribing today: courier-journal.com/tims