False positives as troublesome as negatives

In the high-stakes world of coronavirus testing, one mistake has taken center stage: the dreaded false negative, wherein a test mistakenly deems an infected person to be virusfree.

These troublesome results, experts have said, can deprive a person of treatment and embolden them to mingle with others, hastening the spread of disease.

But false negatives are not the only errors bedeviling coronavirus diagnostics. False positives, which incorrectly identify a healthy person as infected by the virus, can have serious consequences as well, especially in places where the virus is scarce.

Although false positives may seem relatively harmless in comparison with their false-negative cousins, “people can absolutely get hurt,” said Dr. Benjamin Mazer, a pathologist and diagnostics expert at Johns HopkinsUniversity.

False positives are generally very rare among tests that have been vetted by the Food and Drug Administration. But any test can be plagued by contamination, mishandling or technical glitches, leading a device to spot the coronaviruswhere it is not.

Some rapid tests, which forgo sophisticated laboratory equipment and can deliver results in under an hour, have been criticized for returning high numbers of false positives, especially whenused to screenpeople without symptoms. Even laboratory tests that rely on a very reliable technique called polymerase chain reaction, or PCR, have been known to return the occasional false positive.

In places where the virus is relatively scarce, false positives may even outnumber actual positives — eroding trust in tests and, under some circumstances,

A positive result on a coronavirus test sets off a cascade of consequences. According to guidelines published by the Centers for Disease Control and Prevention, people who test positive should immediately isolate for at least 10 days after their symptoms start (if they experience symptoms at all).

That is 10 days spent away fromfriends and family, and 10 days of potential productivity in a school or workplace lost.

Under certain circumstances, a false positive could seed new coronavirus cases. Crowded facilities, such as nursing homes, might isolate coronavirus-positive people together.

In a worst-case scenario, “if someone isn’t actually positive, we could be sending them to their death,” said Dr. Valerie Fitzhugh, a pathologist at Rutgers University.

In early October, officials in Nevada ordered the state’s nursing homes to halt the use of two rapid coronavirus tests until further notice, citing concerns about accuracy. (Nearly 40% of the country’s known COVID-19 deaths have come from nursing homes, according to an analysis by The New York Times.) A rash of false positives had been uncovered, according to the directive, which had risked placing uninfected residents into units with infected people, giving the coronavirusmore opportunity to spread. Such a scenario “could result in causing harm to a population that we have collectively worked so hard to protect,” officials noted in a followup directive.

Under pressure from the federal government, Nevada officials revoked the rapid testing prohibition Oct. 9.

False positives can also be disastrous from a treatment standpoint, said Linoj Samuel, a clinical microbiologist at Henry Ford Health System in Detroit. People with the flu or COVID-19, for example, often show similar symptoms butmay only be tested for one of them at a time. If a patient is given an incorrect diagnosis ofCOVID-19, that person could be deprived of treatment that could alleviate their illness or be given a costly therapy that does little to speed

Mounting evidence suggests that most people who have fought off the coronavirus once retain a degree of immunity that can thwart the virus a second time. Scientists do not know how long this immunity lasts or how it varies from person to person, but theCDChas issued guidelines that note that reinfection is unlikely within 90 days. People who don’t have symptoms within this time windowdo not need to be tested again, the agency has said.

That could be helpful advice for peoplewhoactually contracted the coronavirus. But people who unknowingly receive a false positive on a test might be misled into thinking that they are invincible, said Saskia Popescu, an infection prevention expert at George Mason University.

However, if these people never actually encountered the virus, they would be just as vulnerable as before — if not more, should they choose to forgo masks or physical distancing after assuming their bodies are already bolstered against the virus.

Dr. Catherine O’Neal, an infectious disease specialist at Louisiana State University, expressed similar concerns when discussing a false-positive result given to University of Alabama football coach Nick Saban this month. Should someone be dropped from routine testing, O’Neal said, they would not just imperil their own health. A false positive could put an entire group of close contacts at risk.

With so many of the long-term consequences of coronavirus infections unknown, a positive result can understandably take an emotional toll on the person who receives it, Fitzhugh said.

“You hear that news, and the first thing you think is, ‘I could die,’ ” she said.

On a recent call with Adm. Brett Giroir, who has led the nation’s coronavirus testing efforts, Ruth Katz, senior vice president of policy at LeadingAge, an association of nonprofit providers of aging services, noted that the repercussions of false positives had been a source of extra heartache among residents and staff members.

Mazer noted that false positives on coronavirus tests were likely to evoke some of the same stressful reactions as when other types of clinical screens like mammograms, which search for signs of breast cancer, return incorrect results.

“We see the anxiety, the frustration,” he said.